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.

One of the reasons an email address might not be working is because it was automatically suppressed by our mail server after a "hard-bounce". A hard bounce usually means that there is a permanent error with that address and there is no point sending further emails.