What the message looks like
554 5.4.7 [internal] message timeout (exceeded max time, last transfail: 421 4.7.0 [TSS04] ...)550 5.4.300 Message expired4.4.7 Message expired
What it means
The sending server kept retrying the email after temporary errors, but it still couldn't deliver it before its time limit ran out, so it gave up. The standard name for this status is "delivery time expired". Most servers retry for between one and five days.
The bounce often includes the last temporary error the server got, such as a deferral, a timeout or a full mailbox. That last error is the real reason.
If you're the recipient
The email won't arrive now, so ask the sender to send it again. If the last error was a full mailbox or a problem with your own mail server, fix that first.
If you're the sender
Look for the last error in the bounce, and look it up. A Yahoo TSS04 deferral means Yahoo slowed your sending. A connection timeout means the recipient's server was unreachable. Once it's fixed, send the email again.
If you use addy.io
If a forwarded email expires, it appears on your Failed Deliveries page with the last error from your inbox provider. If you store failed deliveries, you may be able to resend it once the problem is fixed.