All emails to google accounts being blocked - how do I resolve from exchange online?

Question

Outbound mail from Exchange Online domain is being rejected by Gmail with 550 5.7.350 / 550 5.7.1 "message is likely unsolicited mail". SPF, DKIM, DMARC and reverse DNS all pass. Port25 verification confirms SPF pass, DKIM pass and iprev pass.

Any ideas how to fix this please?

Answer 1

Hi Matt

Based on my research, the error 550 5.7.350 / 5.7.1 “message is likely unsolicited mail” is typically returned by the recipient system. Even when SPF, DKIM, DMARC, and reverse DNS pass successfully, Gmail can still apply additional filtering based on factors such as sender reputation, sending patterns, or message content.

In this type of scenario, the issue is often associated with how the message is being evaluated on the recipient side rather than a mail flow problem within Exchange Online.

You may consider reviewing a few areas:

• Whether there has been any recent change in sending volume or pattern
• Message content (for example, links, formatting, or repetitive content)
• Sender or domain reputation based on recent activity
• Any unusual activity within the tenant (for example, compromised accounts)

Exchange Online also includes outbound protection and limits, which can affect sending behavior if certain thresholds or patterns are observed.

For reference, you may find the following documentation useful:

Fix NDR error 550 5.7.1 in Exchange Online - Exchange | Microsoft Learn

Gmail detects my @outlook emails as unsolicited and become undeliverable - Microsoft Q&A

Exchange Online limits - Service Descriptions | Microsoft Learn

As next steps, it may help to review the message headers and run a message trace in Exchange Online, then check Microsoft 365 Defender (Email & collaboration reports) for any unusual activity. You could also try sending test messages with simplified content to Gmail recipients to see if the behavior changes. If the issue persists, reviewing Google Postmaster tools or working with the recipient side may provide additional insight, since the rejection is occurring on Gmail’s side.

I hope this information helps and if you have any questions, please feel free to ask via comment section.

---

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.