When setting up forwarding rules to get your emails into Trello, you might receive a bounced message in your inbox that looks like this.
Your message wasn't delivered because the recipient's email provider rejected it
Although the error makes it seem like SendBoard is rejecting the forwarded emails, this problem is more likely related to a setting with Microsoft 365 (formerly Office 365).
As part of a strict security policy, certain Microsoft 365 accounts will default to block automatic email forwarding. With external forwarding being blocked by your mail server, there is no way for your incoming emails to reach SendBoard & Trello. In these cases when a message is detected as automatically forwarded, and the outbound spam filter policy blocks that activity, the error message is returned to the sender. This is a more specific error that points towards the real problem.
Remote Server returned ‘550 5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS(7555)’
So if you're seeing the above errors, you'll need take an additional step to complete the setup process. If external forwarding is disabled for your Microsoft 365 account, contact your Microsoft 365 administrator and ask them to allow automatic external forwarding for any email addresses that you want to use with SendBoard.
This article provides more information on the changes that need to be made: Control automatic external email forwarding in Microsoft 365