Sometimes emails you send from Trello will fail to reach a recipient because of an error with the email address or the receiving mailbox. If the error message reported back to our mail server indicates that it is a permanent error (hard bounce) we will mark that email address as inactive in order to prevent further failures.
If you try and send an email to an inactive email address, you'll receive an error message at the time of sending. This gives you an opportunity to correct the address if there was a typing error, or replace the address with an alternative.
In some cases, the error with the receiving mailbox might be resolved after some time has passed. If you would like us to reactivate an address that has had errors in the past, please reach out to our support team (firstname.lastname@example.org) with the email address that you would like to reactivate.