Automatic email forwarding rejected in Outlook

Setting up a custom email address in SendBoard requires that you setup a redirect rule that automatically sends emails from your mailbox to your Trello board.

If you receive a bounced message in your inbox that looks like this, you

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.

The text of the bounce error which points towards the real problem will include:

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.
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This Microsoft article provides more information on the changes that need to be made: Control automatic external email forwarding in Microsoft 365