Summary
Emails with email addresses in the subject line are being blocked by Office 365 due to a combination of factors. These include aggressive spam filters, customizable mail flow rules, Microsoft's Smart Screen, and machine learning algorithms that identify suspicious patterns. Specifically, email providers flag email addresses or URLs in the subject line as spam indicators, and Office 365 leverages Exchange Online Protection (EOP) and Advanced Threat Protection (ATP) to enforce these rules. In some cases emails are accepted and then deleted. While Safe Senders lists can bypass some filters, they are ineffective if the domain is blocked. Experts advise avoiding suspicious content in subject lines and reviewing Exchange admin center rules to mitigate these issues. There has also been a reported issue with emojis in the subject line.
Key findings
- Aggressive Spam Filters: Microsoft's spam filters are aggressive and may block legitimate emails, particularly those with email addresses or URLs in the subject line.
- Customizable Mail Flow Rules: Office 365 admins can customize mail flow rules to block emails with specific patterns, including email addresses in the subject.
- Smart Screen Blocking: Microsoft's Smart Screen technology can block emails after initial acceptance, indicating a deeper analysis flagging the message as spam.
- Machine Learning: Office 365 uses machine learning algorithms to identify and block spam based on various factors, including email addresses in the subject.
- EOP/ATP Filtering: Exchange Online Protection (EOP) and Advanced Threat Protection (ATP) provide anti-spam filtering capabilities leading to the blocking or quarantine of emails with suspicious subjects.
- Suspicious Subject Lines: The presence of email addresses, URLs, or even emojis in the subject line triggers spam filters due to the perception of suspicious content.
Key considerations
- Avoid Suspicious Content: Avoid including email addresses, URLs, and unusual characters or emojis in subject lines to prevent triggering spam filters.
- Review Exchange Admin Center: Regularly review and adjust mail flow rules and anti-spam policies in the Exchange admin center to ensure they are not overly restrictive.
- Monitor Blocked Emails: Monitor blocked emails and quarantine folders to identify any legitimate emails that are being incorrectly flagged as spam.
- Safe Senders Lists Limitations: Be aware that Safe Senders lists may not always bypass spam filters, especially if the sending domain is already blocked.
- Escalate Support Tickets: For emails accepted then deleted, open a support ticket with Microsoft and escalate as necessary to resolve issues related to Smart Screen.
- Consider alternative subject lines: Consider alternative subject lines to email addresses, URLS and emojis to avoid being flagged as spam.
What email marketers say
8 marketer opinions
Emails with email addresses in the subject line are being blocked by Office 365 primarily because they are often flagged as spam. This can be due to aggressive spam filters, custom or default mail flow rules, or machine learning algorithms identifying suspicious patterns. Email providers often consider email addresses (or URLs) in the subject line as a sign of spam, a common tactic used by spammers to bypass filters. Additionally, some specific blockers, such as emojis, can also trigger rejections. It is generally considered bad practice to include email addresses in the subject line.
Key opinions
- Aggressive Spam Filters: Microsoft's spam filters are aggressive and may block legitimate emails with email addresses in the subject.
- Custom Mail Flow Rules: Office 365 admins can create custom mail flow rules that block emails with specific patterns in the subject line, including email addresses.
- Machine Learning: Office 365 uses machine learning to identify and block spam, potentially flagging emails with email addresses in the subject.
- Email Address as Spam Signal: Email providers often consider email addresses (or URLs) in the subject line as a spam indicator.
- Avoid Subject Line: Subject lines with an email address or anything that appears suspicious is a red flag.
Key considerations
- Check Exchange Admin Center: Review the Exchange admin center for any custom or default rules that may be blocking these emails.
- Avoid Email Addresses in Subject: Refrain from including email addresses in the subject line to prevent triggering spam filters.
- Monitor Blocked Emails: Regularly monitor blocked emails to identify any legitimate emails being incorrectly flagged.
- Consider Safe Senders Lists: Use Safe Senders Lists to bypass some filters, however this will not bypass if the domain is blocked.
Marketer view
Marketer from Email Geeks shares they found a blocker with Outlook recently related to emojis in the subject line.
3 Aug 2023 - Email Geeks
Marketer view
Email marketer from Spiceworks Community suggests that Office 365 might have a default or custom rule that identifies and blocks emails with specific patterns in the subject line, including email addresses. He recommends checking the Exchange admin center for any such rules.
16 Dec 2023 - Spiceworks Community
What the experts say
3 expert opinions
Emails with email addresses in the subject line are likely being blocked by Office 365 because they are flagged as spam. Including suspicious elements like URLs or email addresses in the subject line is a common red flag for spam filters. In some cases, the mail may be accepted initially but then deleted by Microsoft Smart Screen. For this, you may need to open a support ticket.
Key opinions
- Spam Flag: Subject lines with email addresses are highly likely to be flagged as spam.
- Suspicious Elements: URLs and email addresses in subject lines are red flags for spam filters.
- Microsoft Smart Screen: Mail accepted and then deleted indicates a Microsoft Smart Screen issue.
Key considerations
- Avoid Suspicious Content: Avoid using email addresses or URLs in subject lines.
- Support Ticket: If mail is accepted but then deleted, open a support ticket with Microsoft and escalate.
- Creative Subject Lines: Prioritize creative subject lines while avoiding elements that may trigger spam filters.
Expert view
Expert from Word to the Wise explains that using creative subject lines is important, but avoid anything that looks suspicious like a URL or email address. These are often red flags for spam filters.
8 Jun 2025 - Word to the Wise
Expert view
Expert from Spam Resource explains that a subject line containing an email address is highly likely to be flagged as spam and blocked.
14 May 2024 - Spam Resource
What the documentation says
4 technical articles
Emails with email addresses in the subject line are being blocked by Office 365 primarily due to anti-spam filtering capabilities provided by Exchange Online Protection (EOP) and Office 365 Advanced Threat Protection (ATP). Administrators can configure policies and mail flow rules (transport rules) to block or quarantine messages based on various criteria, including the presence of email-like patterns in the subject line. While Connection Filter policies, which use IP lists, are not related, the presence of email addresses in the subject makes the message appear suspicious. Safe Senders lists may bypass some filters, but not if the domain is blocked.
Key findings
- Anti-Spam Filtering: EOP and Office 365 ATP provide anti-spam filtering capabilities that can block emails with email addresses in the subject line.
- Mail Flow Rules: Mail flow rules in Exchange Online can be configured to examine subject lines and block emails containing email addresses.
- Suspicious Appearance: Email addresses in the subject line appear suspicious to email providers.
- Safe Senders Limitation: Safe Senders lists may bypass some filters but not if the domain is blocked.
Key considerations
- Configure Policies: Administrators should configure policies to manage anti-spam settings effectively.
- Review Mail Flow Rules: Review mail flow rules to ensure they are appropriately configured to handle emails with email addresses in the subject line.
- Avoid Suspicious Subject Lines: Avoid using email addresses in the subject line to prevent being flagged as spam.
- Consider Safe Senders: Utilize Safe Senders lists with caution, understanding their limitations regarding blocked domains.
Technical article
Documentation from Microsoft Learn explains that mail flow rules (also known as transport rules) in Exchange Online can be configured to examine message headers and content (including the subject line) and take action based on specified conditions. An administrator could create a rule to block or quarantine emails with email addresses in the subject line.
15 Jun 2025 - Microsoft Learn
Technical article
Documentation from Microsoft Learn explains that Exchange Online Protection (EOP) and Office 365 Advanced Threat Protection (ATP) provide anti-spam filtering capabilities. Admins can configure policies to block or quarantine messages based on various criteria, and one possible reason for blocking emails with email addresses in the subject line could be a custom rule or a default rule that is triggered by the presence of email-like patterns in the subject.
25 Nov 2021 - Microsoft Learn
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