Why are transactional emails sent via PostMarkApp being blocked by Office365 despite correct SPF, DKIM, and DMARC settings?

Email marketer from StackOverflow responds that Office 365 may block emails if the sending IP address has a poor reputation. Microsoft uses various blocklists, and if the IP is on one, emails can be rejected even if SPF, DKIM, and DMARC are configured correctly. Also suggested checking the sending IP against common blacklist databases to ensure it isn't listed anywhere else.

Email marketer from EmailOnAcid responds that some issues stem from how the authentication protocols are implemented. Even if they are technically "set up," there may be configuration errors. It recommends using a tool to validate that the SPF, DKIM, and DMARC records are correctly formatted and resolving properly.

Email marketer from Email Geeks explains that the issue may not be the mail filter itself, but custom rules organizations put into place in O365, such as strict DMARC alignment, which can quarantine non-compliant messages.

Email marketer from SocketLabs discusses Microsoft's Smart Network Data Services (SNDS) program. By enrolling in SNDS, you can monitor the health of your sending IP address as seen by Microsoft. This data can provide insights into why emails are being blocked or junked, such as high spam complaint rates or low sender reputation.

Email marketer from Reddit explains that with shared sending infrastructure (like Postmark), the reputation of the shared IP address can affect deliverability. If other users on the same IP are sending spam, it can negatively impact everyone's deliverability, including legitimate emails. Recommend investigating dedicated IP options to isolate your sending reputation.

Email marketer from Email Geeks shares that undelivered emails despite successful delivery confirmations often occur with O365 due to content filter quarantines. The IT department using the O365 instance can use Advanced Threat Hunting to find quarantined messages and reasons, often due to custom rules. Providing friendly from and recipient email addresses usually helps locate the message.

Email marketer from SparkPost notes if a new IP address is being used this needs to be correctly warmed up so as to not get marked as spam.

Email marketer from Mailjet suggests that Office 365 may block emails due to the content triggering spam filters. Elements like excessive use of spam trigger words, poor HTML formatting, or a high image-to-text ratio can cause issues, even with correct authentication. Suggests reviewing the content and using a spam testing tool.

Email marketer from DNSQueries tool shares that having a correctly configured reverse DNS (PTR) record is crucial for email deliverability. The PTR record maps an IP address back to a domain name. If the PTR record doesn't match the sending domain, Office 365 may flag the email as suspicious, even with proper SPF, DKIM, and DMARC.

Email marketer from Reddit highlights that Office 365 uses aggressive throttling policies to protect its infrastructure from abuse. If a sending IP is sending too many emails too quickly, Office 365 may temporarily block the IP, even if the emails are legitimate and properly authenticated. Need to gradually increase the sending volume.

Email marketer from Gmass mentions that it can sometimes be caused by being on a RBL, recommends checking your IP and domain reputation using a tool like MultiRBL.

Expert from Email Geeks suggests examining the PCL score (phishing suspicion) or checking the customer's website for potential compromise and malware serving, assuming the customer sends opt-in emails.

Expert from Email Geeks suggests checking successful deliveries to own O365 addresses for BCL/SCL levels. If high (e.g., 9), Microsoft might allow tenants to delete those emails before inbox delivery, requiring improvement of the BCL score or tenant filter changes.

Expert from Spam Resource, John Levine, responds that Microsoft's filtering is complex and can be affected by factors beyond authentication. Shared IP reputation, content filtering, and user complaints all play a role. Even with correct SPF, DKIM, and DMARC, a sudden spike in sending volume or negative feedback can trigger blocks.

Expert from Word to the Wise, Laura Atkins, shares that SNDS (Smart Network Data Services) provides valuable insight into how Microsoft views your sending IP. Using SNDS can help in identifying if Microsoft is flagging your IP for spam.

Documentation from Microsoft Learn explains that several factors can cause emails to be blocked by Office 365, even with correct authentication. These include IP address reputation, domain reputation, spam filter settings, and content of the email itself triggering spam filters.

Documentation from Spamhaus explains that they provide blocklists and reputation lookups for both IP addresses and domain names. If either the sending IP or domain is listed on a Spamhaus blocklist, Office 365 is highly likely to block the emails, regardless of SPF, DKIM, and DMARC.

Documentation from Postmark Support recommends checking feedback loops (FBLs) to identify spam complaints from Office 365 users. High complaint rates can lead to blocking, even with proper authentication. Also, suggests monitoring Postmark's delivery statistics for any error messages or delivery failures specific to Office 365.

Documentation from Microsoft Learn notes that Microsoft uses a sender reputation system that evaluates various factors, including sending volume, email content, and user complaints, to determine the reputation of a sending IP address or domain. A poor sender reputation can lead to blocked or junked emails, regardless of authentication protocols.

Documentation from RFC explains that DMARC policies, particularly 'p=reject', can cause emails to be blocked if they fail SPF and DKIM checks. Even if the checks are generally passing, subtle configuration issues or inconsistencies can lead to intermittent failures and blocked emails. Check the DMARC reports to diagnose.