Why does O365 mark emails from my domain sent via a third-party ESP as spam?

Marketer from Email Geeks suggests that Advanced Threat Protection (ATP) add-on could be the cause, as it's designed to counter phishing and may behave in the described manner without specific tweaking, reinforcing the idea of a site-specific issue.

Email marketer from GlockApps shares that not performing email placement testing before sending campaigns can lead to deliverability issues. Testing helps identify potential problems with content or authentication that might cause emails to be marked as spam by O365.

Email marketer from Postmark Blog answers that poor list hygiene practices (e.g., sending to old or unengaged email addresses) can lead to high bounce rates and spam complaints, which negatively impact your sender reputation with O365.

Email marketer from Litmus explains that accessibility issues, such as missing alt text on images or poor color contrast, can trigger spam filters. While not a direct cause, it can contribute to a lower engagement rate, affecting overall deliverability.

Marketer from Email Geeks suggests that if there is a Mimecast in front of that O365, the user might have the impersonation rule active.

Email marketer from Sendinblue Blog answers that not being registered with feedback loops (FBLs) can negatively impact deliverability. FBLs allow senders to identify and remove subscribers who mark emails as spam, improving sender reputation and deliverability with O365.

Email marketer from Mailjet Blog shares that the content of the email can trigger spam filters in O365. Using spam trigger words, having a high image-to-text ratio, or including suspicious links can all contribute to emails being marked as spam.

Email marketer from Reddit explains that if your ESP uses shared IP addresses, the actions of other senders using the same IP can affect your deliverability. If other senders are engaging in spammy practices, O365 might block emails from the entire IP range.

Email marketer from DNSstuff explains that a missing or incorrect reverse DNS (PTR) record can cause issues with email deliverability. A PTR record maps an IP address to a domain name, and its absence can make it harder for O365 to verify the legitimacy of the sending server.

Email marketer from ReturnPath notes that if your sending IP or domain is on a blocklist, O365 will likely mark your emails as spam. Monitoring blocklists and taking steps to be removed is crucial for maintaining deliverability.

Email marketer from SparkPost Blog explains that the IP reputation of the sending server used by the ESP can significantly impact deliverability. If the IP address has a poor reputation due to previous spam activity, O365 is more likely to mark emails as spam.

Email marketer from Email Marketing Forum shares that O365 heavily relies on sender reputation. If your domain or sending IP has a poor reputation due to low engagement rates or high complaint rates, your emails are more likely to end up in the spam folder.

Expert from Word to the Wise, Steve Jones, explains Microsoft utilizes its SmartScreen filter which adapts and learns based on user interactions and data signals. If enough recipients mark emails from a particular sender or ESP as junk, SmartScreen learns to filter those emails automatically for other users. This can happen even if authentication is technically correct.

Expert from Word to the Wise, Laura Atkins, responds that Office 365/Microsoft is particularly picky about authentication. Issues often arise when organizations move to O365 and fail to ensure their SPF, DKIM, and DMARC records are correctly configured and aligned, especially when using a third-party ESP. Even small misconfigurations can lead to deliverability problems. Microsoft may also have internal reputation metrics that trigger filtering based on volume or content, even if authentication passes.

Expert from Email Geeks suggests a possible configuration setting that treats all mail using a domain but originating from a different SMTP server as forged and that it sounds like a local config issue related to anti-forgery processing. Also states that MS is doing this because the domain told them to.

Expert from Email Geeks asks about the Authentication results, suggesting that Microsoft might be processing mail differently when the domain is hosted with them but sent from an external source.

Documentation from Microsoft Learn details that DMARC policies dictate how receiving mail servers (like O365) should handle emails that fail SPF and DKIM checks. A DMARC policy that is too strict (e.g., set to reject) can cause legitimate emails sent through a third-party ESP to be marked as spam if they don't pass authentication.

Documentation from Microsoft Learn explains that an incorrect or missing SPF record can cause O365 to mark emails as spam, especially when sent from a third-party ESP. It is crucial to properly configure the SPF record to include the sending sources (ESP servers) to authenticate the emails correctly.

Documentation from RFC editor explains that failing to properly verify domain ownership can cause issues with authentication protocols like SPF and DKIM. O365 requires proper verification to ensure that the sender is authorized to send emails on behalf of the domain.

Documentation from Microsoft Learn highlights that DomainKeys Identified Mail (DKIM) is vital for authenticating outbound email. If DKIM is not properly set up for your domain with the third-party ESP, O365 might flag the emails as spam. Implementing DKIM ensures that emails are signed and verified as legitimate.