Why are emails with email addresses in the subject line being blocked by Office 365?

Marketer from Email Geeks shares they found a blocker with Outlook recently related to emojis in the subject line.

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.

Email marketer from StackOverflow says that subject lines that resemble email addresses are often flagged as spam. If possible change the subject line to make it seem less suspicious.

Email marketer from TechForum mentions that Office 365 uses machine learning to identify and block spam, and it is possible that emails with email addresses in the subject line are being flagged as spam due to this algorithm.

Email marketer from answers.microsoft.com explains that Microsoft's spam filters are aggressive and can sometimes block legitimate emails. Including email addresses in the subject line could trigger these filters, especially if the content is also considered suspicious.

Email marketer from Reddit mentions that some email providers, including Office 365, have spam filters that specifically look for email addresses or URLs in the subject line as a sign of spam. This is because spammers often use these tactics to try to bypass filters.

Email marketer from answers.microsoft.com responds that Emails are being blocked by Outlook for several reasons. Email address in subject line may appear spammy and as a security measure Outlook can block those emails.

Marketer from Email Geeks suggests that emails in subjects is a bad idea and that admins can customize their mail flow rules to suit their business and regulatory needs.

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.

Expert from Spam Resource explains that a subject line containing an email address is highly likely to be flagged as spam and blocked.

Expert from Email Geeks explains that mail being accepted but then deleted is a Microsoft Smart Screen issue and recommends opening a support ticket and escalating until someone can help, describing the issue as mail accepted but never delivered.

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.

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.

Documentation from Microsoft Support explains that Safe Senders lists may bypass some filters but will not bypass if the domain is blocked. The email address in the subject line may appear suspicious to email providers and cause the message to be undelivered.

Documentation from Microsoft Learn explains that Connection Filter policy in Microsoft 365 uses allowed and blocked IP lists. So it isn't related to the email addresses in the subject line.