I'm facing a frustrating issue with Office 365 that I hope someone can help me solve. Both Tenant A and Tenant B are using the same third-party mail filter, and when Tenant A tries to send an email to Tenant B, the process gets stuck in a loop. Office 365 sends the email to the mail filter, which then forwards it to the correct Outlook host. However, it ends up bouncing around within O365 and gets sent back to the mail filter repeatedly. The tracing for Tenant A shows the email being delivered multiple times to the external filter, while nothing shows up for Tenant B. If I directly send an email to "tenantb.mail.protection.outlook.com" using a script, it gets accepted but still results in the same loop. Microsoft claims it's an issue with the mail filter provider, but their logs indicate the mail performs the same loop back to O365. We've tried setting up inbound connectors specifying the mail filter's IPs without success. Mail from outside O365 works fine for Tenant B; it's just Tenant A that can't get through. Any insights on what might be going wrong?
2 Answers
Have you checked the email headers? They might provide some insights into where the issue is occurring in the email routing process.
I had a similar issue last year with a specific inbound connector causing a loop like yours. We found that using an inbound connector named "Scan to email" created attribution problems. If neither tenant is using a hybrid setup, they don’t recommend using on-premises connectors. Switching to Partner connectors fixed everything for us!
Thanks for the tip! It sounds like your situation is quite similar. I found a link about a 'Hop Count Exceeded' error related to filtering products, even though ours might be different. We're planning to revise the inbound connectors to see if that helps.