We had this working for several months until early January of this year. I've opened an incident and it keeps on getting closed or abandoned by Microsoft and can't seem to get any traction for support as an M365 admin through Microsoft's channels, we're a month into the problem.
All the e-mails are forwarded through an Exchange Online account from our local Exchange server, they all appear there, if I trace the messages in Microsoft Exchange, I can see them being accepted without error from Microsoft Teams, but from that point, it is a mystery. It uses a different domain than the other e-mails that are routing through, but that domain has been approved for the channel and nothing is blocking it in our Exchange or Teams tenant configuration.
Support has me go through the same steps, trace it, re-create the channel SMTP address, check the same settings, then I'm told I'll get a follow-up or to run more traces and provide the information. I'm at a loss as to how to trace what happens once Teams accepts the e-mail, from Exchange and Exchange Online, everything looks fine.
Thanks for whatever help or suggestions you have. Like I said, we had it working for months, hadn't changed anything, and it just stopped-- it is part of a fairly critical workflow for the organization.