Microsoft Mandates DMARC for Bulk Email by May2025

From May 5, 2025, high-volume senders must enable SPF, DKIM, and DMARC, bringing Microsoft in line with Google and Yahoo to curb spoofing and Phishing.
By
CTM360 Team
April 21, 2025
1 mins read
Microsoft Mandates DMARC for Bulk Email by May2025
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What’s on this page

Overview
CTM360’s observation of the trend
Recommendations

Overview

The fight against email-based threats is intensifying. Following the lead of Google and Yahoo, Microsoft has officially announced the mandatory implementation of email authentication protocols, SPF, DKIM, and DMARC, for high-volume email senders, effective May 5, 2025. Domains sending over 5,000 daily emails to Microsoft's platforms, including Outlook.com, Hotmail.com, and Live.com, will be required to authenticate their messages. Initially, non-compliant messages will be redirected to recipients' Junk folders, with eventual total rejection expected if compliance isn't achieved. This enforcement represents a critical step in securing global email communications from spoofing and phishing threats (Microsoft Tech Community).

Understanding Email Authentication

Email authentication has become essential in combating increasingly sophisticated phishing and spoofing attacks. Three core standards have been adopted widely:

  • Sender Policy Framework (SPF: RFC 7208): Verifies the legitimacy of the sending mail server, ensuring messages originate from authorized infrastructure. SPF helps prevent sender address forgery by defining authorized sending sources in DNS records.
  • DomainKeys Identified Mail (DKIM: RFC 6376): Cryptographically signs email messages, allowing recipients to confirm message content hasn't been altered in transit. DKIM leverages public-private key pairs to ensure message integrity and authenticity.
  • Domain-based Message Authentication, Reporting, and Conformance (DMARC: RFC 7489): Combines SPF and DKIM to enforce domain alignment, verify authenticity, and provide reporting mechanisms for senders to track email usage and potential abuse. DMARC is recommended to be implemented in three incremental stages: initial monitoring (p=none), intermediate quarantining of suspicious messages to test the policy impact (p=quarantine), and full rejection of unauthorized emails to achieve DMARC compliance (p=reject). 

Email Content and Delivery Best Practice Guidance

To help the email ecosystem thrive and ensure that legitimate communications reach users' inboxes, major providers like Google, Yahoo, and Microsoft have released a unified set of technical and content-based requirements. This section consolidates these guidelines into a single resource for senders seeking to avoid spam filtering and maintain high deliverability rates.

1. Message Headers & Structural Integrity

Best Practice Description
Valid and consistent From header Use a single, clear email identity. Avoid multiple addresses in From and misleading sender names.
Aligned Reply-To domain Ensure Reply-To reflects the same domain or purpose as From.
Unique and compliant Message-ID Follow RFC 5322 formatting. Avoid duplicate or malformed IDs.
Proper MIME structure and header syntax Messages must conform to standard email formatting. Avoid malformed headers and nested MIME issues.
Avoid forged headers Do not spoof or misuse headers associated with major domains (e.g., gmail.com, outlook.com).

2. Content Hygiene and Formatting

Best Practice Description
Avoid deceptive subject lines Refrain from using misleading tags like "RE:" or "FWD:" unless applicable.
Balanced text-to-image ratio Do not send image-only emails. Include meaningful text with alt text for images.
Email size < 100 KB Ensure the email body stays within standard size limits (typically under 100 KB) to avoid clipping in mail clients. This refers only to the message content and does not include attachments.
Professional formatting Avoid ALL CAPS, excessive punctuation, invisible text, and non-standard fonts.
Exclude scripts and forms Embedded forms or JavaScript will trigger spam or phishing filters.
Consistent branding and tone Use recognizable logos, colors, and sender names to build trust.

3. Infrastructure and Technical Configuration

Best Practice Description
SPF configuration Define valid authorized sending sources in the domain’s SPF record. Must align with the domain in the From header.
DKIM configuration Cryptographically sign messages using DKIM with a domain that matches the From address.
DMARC configuration Publish a DMARC record at minimum with p=none. Domain alignment with SPF or DKIM is required to pass DMARC checks.
Valid PTR (reverse DNS) records The sending IP address must resolve to a valid hostname that maps back to the same IP address.
TLS for outbound SMTP TLS is mandatory for Gmail. Senders without encryption may be rejected.
SPF lookup limit adherence Keep SPF DNS lookups ≤ 10. Microsoft enforces this.
IP/domain warming Gradually increase send volume from new IPs or domains to build a reputation.
Consistent sending patterns Avoid sending bursts or erratic volumes. Maintain daily volume stability.
ARC headers for forwarded email ARC ensures original authentication results are preserved through intermediaries.

4. Recipient List Management

Best Practice Description
Explicit opt-in only Do not use purchased lists. Only email users who have explicitly subscribed.
Functional one-click unsubscribe Add RFC-compliant headers:
List-Unsubscribe: <https://domain/unsub?id=xyz>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
Unsubscribe fulfillment within 48 hours Honor removal requests promptly to reduce complaints.
List cleaning and bounce management Regularly purge unengaged, bounced, or invalid addresses.
Segmentation by message type Separate promotional, transactional, and notification content using distinct sender identities.

5. Engagement and Complaint Monitoring

Tool / Method Purpose
Google Postmaster Tools Monitor domain/IP reputation, spam rate, and user engagement.
Yahoo Complaint Feedback Loop (CFL) Receive ARF reports for complaints and unsubscribe flagged users.
Maintain a complaint rate < 0.3% High complaint rates trigger deliverability throttling and domain penalties.
Monitor bounce and open rates Use these to assess the health of your lists and campaigns.

6. Summary Recommendations

  1. Align headers and domains with clear, professional identities.
  2. Respect opt-in and unsubscribe behaviors with transparent mechanisms.
  3. Structure content to be clean, concise, and free of deceptive or spammy characteristics.
  4. Maintain technical hygiene through DNS, TLS, SPF limits, and ARC usage.
  5. Monitor sender reputation and user engagement continuously.
  6. Ensure SPF and DKIM are properly configured and aligned with the domain in the From header, and publish a DMARC record with at least p=none to begin monitoring and enforcement.

Immediate Impact and Risks of Non - compliance

As of May 2025, domains that fail SPF or DKIM checks or lack a correctly configured DMARC policy with alignment will risk having their emails marked as spam or not delivered at all. Misalignment occurs when the domain used in the message's "From" address doesn't match the domains authenticated by SPF or DKIM.

Organizations failing to comply face significant risks, including diminished deliverability rates, compromised customer trust, and increased susceptibility to impersonation attacks. These impacts directly affect an organization's reputation, customer engagement, and potentially, its revenue.

Actionable Recommendations for Immediate Implementation

To effectively prepare for these mandatory standards, organizations should:

  • Audit current DNS records: Utilize tools such as "dig" or Google DNS to verify SPF, DKIM, and DMARC records.
  • Begin with Monitoring (p=none): Initially deploy DMARC in monitoring mode to understand email flows and detect anomalies without risking legitimate email delivery.
  • Gradually enforce stricter policies: Move from quarantine to full rejection while monitoring.
  • Ensure domain alignment: "From" domain must match what’s authenticated via SPF or DKIM.
  • Maintain email hygiene: Clean lists, include a clear opt-out option, and avoid using misleading subject lines or headers.

Start Your DMARC Journey with CTM360 Free Community Edition

To support organizations navigating these changes, CTM360 offers a complimentary zero-cost Community Edition platform. It allows comprehensive monitoring, management, and enhancement of your DMARC records and email authentication setup. This proactive approach helps organizations reduce risks associated with impersonation attacks and maintain reliable email communication.

Join CTM360 Community Edition today, no hidden costs, simply real security.

Reference:

Disclaimer:

The information contained in this document is meant to provide general guidance and brief information to the intended recipient pertaining to the incident and recommended action. Therefore, this information is provided "as is" without warranties of any kind, express or implied, including accuracy, timeliness, and completeness.

Consequently, under NO condition shall CTM360®, its related partners, directors, principals, agents, or employees be liable for any direct, indirect, accidental, special, exemplary, punitive, consequential, or other damages or claims whatsoever including, but not limited to loss of data, loss in profits/business, network disruption...etc., arisina out of or in connection with this advisory.

For more information: Email: monitor@ctm360.com Tel: (973) 77 360 360

Recent Blogs

A Browser-in-the-Browser (BitB) attack is a sophisticated cyber threat involving injecting malicious code into a victim's web browser.

Threat Overview:

This code creates a secondary browser within the victim's existing browser, allowing the attacker to manipulate web content, intercept network requests, and potentially gain control over the victim's browser and system. Understanding the BitB attack is crucial for organizations to develop effective mitigation strategies.

screenshot 2023 11 13 at 11 58 48 am

   Once the website is opened, it appears as shown above (this will be the first step).

screenshot 2023 11 13 at 12 02 39 pm

After enforcing the Full-screen display mode, only the inner browser will appear with the customized URL by the attacker.

Real-Case Scenarios of BitB Attacks:

CTM360 recently observed ongoing attack campaigns utilizing the BitB technique targeting ministries and government websites, specifically the interior ministries.

In the previously mentioned scenario, the official website of MOI Singapore remains unaffected and secure. However, the threat actor is carrying out a phishing attack by creating a fake website. Within this fraudulent site, instead of using traditional phishing methods, such as fake forms or malicious content, the attacker employs a fake browser interface within the phishing site, which appears to be the genuine site for MOI. When the victim accesses the site, they are presented with a full-screen display mode of this embedded browser, which tricks them into submitting their sensitive information.

Attack Methodology

To execute a Browser-in-the-Browser (BitB) attack, the attacker employs tactics to lure the user into visiting a malicious or compromised website. This website contains a phishing page hosted on the attacker's server. The phishing page utilizes JavaScript code to create a simulated browser window, simulating the appearance and behavior of a legitimate browser window. Within this simulated window, various types of fraudulent activities can be displayed.

Moreover, the simulated window displays a URL of the attacker's choice, such as https://accounts.google.com or https://login.microsoftonline.com. This is achieved by modifying the simulated address bar of the pop-up window using JavaScript. It may appear to the user that the specified URL is loaded within the pop-up window, but in reality, it is only shown as an image or text. The user may not notice the absence of SSL certificates or other security indicators typically present in a genuine browser window due to the full-screen display mode, which blocks the appearance of the main website’s URL due to the full-screen display model.

If the user falls victim to the BitB attack and enters their login credentials into the fake login form, the information is sent to the attacker's server via an AJAX request or a concealed form submission. Subsequently, the attacker gains access to the user's account on the legitimate service or proceeds with additional malicious activities such as identity theft or account takeover.

Potential Threats

Browser-in-the-Browser (BitB) attacks pose several potential threats and risks to victims. Here are some of the common threats associated with BitB attacks:

  • Data Theft: Attackers can exploit BitB attacks to steal sensitive information, such as login credentials, financial details, personal data, or intellectual property. This stolen data can be used for identity theft, financial fraud, or sold on the dark web.
  • Account Takeover: By manipulating the victim's browser and intercepting login credentials, BitB attacks can lead to unauthorized access to the victim's online accounts. Attackers may gain control over email accounts, social media profiles, online banking, or other services, enabling them to impersonate the victim or perform malicious activities.
  • Malware Distribution: BitB attacks can be used as a vector to distribute malware onto the victim's system. The secondary browser created by the attacker can be used to download and execute malicious software, potentially leading to further compromise of the victim's device and sensitive data.
  • Phishing and Social Engineering: Attackers can utilize BitB attacks to create convincing phishing scenarios. By simulating legitimate websites or services, they trick users into entering their login credentials or other sensitive information, which the attacker then captures and exploits.

Mitigation

The BitB attack is a tricky and risky phishing technique that can trick even careful users and bypass typical security measures. However, there are steps you can take to protect yourself from this attack:

  • Be cautious of full-screen prompts: Exercise caution if a website unexpectedly opens a full-screen prompt or overlay. Take a moment to assess the situation and ensure that you are interacting with a legitimate website before entering any sensitive information.
  • Be vigilant about website URLs: Pay close attention to the URL before entering any sensitive information. Check for any discrepancies or variations in the domain name or spelling that may indicate a phishing site.
  • Pay attention to the details of the pop-up window, such as the size, position, appearance, and behavior of the elements. If something looks off or unusual, you should close the window and report it.
  • Use a security-focused browser extension that can detect and block such phishing attempts automatically.
  • Keep your browser up to date with the latest security patches and update whenever prompted by your browser.
  • Make sure you have 2FA enabled for all of your critical services.

CTM360 is actively monitoring this phishing campaign and taking the necessary action by disrupting the attack and suspending the malicious site/domain. If you encounter any of such malicious sites, please report it to business@ctm360.com.

References

What is ChatGPT?

ChatGPT, an advanced AI language model created by OpenAI, is gaining popularity and attention for its ability to generate human-like responses to natural language input. Trained on large amounts of data, ChatGPT's context comprehension and relevant response generation have made it a popular choice for businesses seeking to enhance customer experience and operations.

Major technology corporations are making significant investments in Artificial Intelligence (AI). Microsoft, for instance, has declared that it will invest $10 billion in OpenAI and intends to merge ChatGPT into its Azure OpenAI suite. This will allow businesses to include AI assets, including DALL-E, a program that generates images, and Codex, which transforms natural language into code, in their technology infrastructure.

While ChatGPT has several benefits for financial institutions, such as improving customer service and automating certain tasks, it also carries some risks that need to be addressed. Major banks and other institutions in the US have banned the use of ChatGPT within the organization. Concerns over sensitive information being put into the chatbot.

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Risks associated with incorporating ChatGPT

Let's delve into the potential risks that are currently being debated regarding the use of ChatGPT:

  1. Data Exposure: One potential risk of using ChatGPT in the workplace is the inadvertent exposure of sensitive data. For example, employees using ChatGPT to generate data insights and analyze large amounts of financial data could unknowingly reveal confidential information while conversing with the AI model, which could lead to breaches of privacy or security. Another known data exposure case observed is Employees could potentially expose private code if they inadvertently include confidential information in the training data. This could occur if an employee includes code snippets that contain sensitive data or proprietary information, such as API keys or login credentials.
  2. Misinformation: ChatGPT can generate inaccurate or biased responses based on its programming and training data. Financial professionals should be cautious while using it to avoid spreading misinformation or relying on unreliable advice. ChatGPT’s current version was only trained on data sets available through 2021. In addition, the tool pulls online data that isn’t always accurate.
  3. Technology Dependency: While ChatGPT offers useful insights for financial decision-making, relying solely on technology may overlook human judgment and intuition. Financial professionals may misunderstand ChatGPT's recommendations or become over-reliant on it. Thus, maintaining a balance between technology and human expertise is crucial.
  4. Privacy Concerns: ChatGPT gathers a lot of personal data that users, unassumingly, might provide. Most AI models need a lot of data to be trained and improved, similarly, organizations might have to process a massive amount of data to train ChatGPT. This can pose a significant risk to individuals and organizations if the information is exposed or used maliciously.

External Risks associated with ChatGPT

  1. Social Engineering: Cybercriminals can use ChatGPT to impersonate individuals or organizations and create highly personalized and convincing phishing emails, making it difficult for victims to detect the attack. This can lead to successful phishing attacks and increase the likelihood of individuals falling for the scam.
  2. Creating malicious scripts and malware: Cybercriminals can train ChatGPT on vast amounts of code to produce undetectable malware strains that can bypass traditional security defenses. By using polymorphic techniques like encryption and obfuscation, this malware can dynamically alter its code and behavior, making it challenging to analyze and identify.

Recommendations:

  • Financial institutions should establish clear policies and guidelines for using ChatGPT in the workplace to safeguard confidential information and mitigate the risks of data exposure.
  • Anonymized data should be used to train an AI model to protect the privacy of individuals and organizations whose data is being used.
  • Specific controls should be applied to how employees use information from ChatGPT in connection with their work.
  • Awareness training should be provided to Employees who have access to ChatGPT on the potential risks associated with the use of the technology, including the risks of data exposure, privacy violations, and ethical concerns.
  • Restricting access to ChatGPT will limit the potential for data exposure and misuse of the technology.

There is an ongoing scam email campaign targeting executives from prominent organizations around the world. These reported email addresses on the domains are genuine email accounts of 'Virgin Media', 'Optimum' & 'Cox communications' users' which are compromised and sold on underground forums. Scammers buy and use such accounts as they have a higher reputation compared to newly created free accounts to bypass email gateways and spam filters and also lower their risk of being detected or tracked back in a bigger BEC attack.

These accounts are compromised by threat actors via phishing emails and malware, and the issue with such accounts will persist as long as end users are not careful enough while dealing with phishing and malicious emails. Based on our previous experience in response with the hosts Virgin Media, Optimum and Cox.net which are TV and cable providers in the UK and USA. They do not investigate email accounts reported to their abuse team without the original email and header information as this is a mandatory requirement as per their abuse policies. However, once this information is provided they do take action by resetting/recovering the reported account of the original user.

Additionally, enabling strict quarantine measures on your email gateway from these domains will make you a harder target and eventually the attackers moving on to another one.

RECOMMENDATIONS

  • As a precautionary measure, all email from private and/or external email addresses which contain the name of your executive staff in the "From" field may flagged.
  • As part of containment, please ensure to quarantine emails from the domain @virginmedia[.]com, @optimum[.]net, @optonline[.]net, @cox[.]net, @cox[.]com
  • As this type of email is tricky, it is highly recommended to conduct phishing awareness training for all employees to avoid potential risks.