You can follow it through the 2 different MTAs (inbound, delivery) and identify where it stopped.
Chances are it's still in some kind of badmail queue (as it could be undeliverable), or it was stopped in its way down to the mail server by another device... Still the problem seems to have occurred within the SMTP conversation/handover from a MTA to the other.
Check the message size whether it has just reached the default limit.
Did you relay via other server (SMTP and mai box are not the same server)? If you did not send mail direct from your mail server, you may get that error. That is what we called "Challenge Response".
The Challenge Response system requires senders from external domains to authorize their email address for future communication with your mail server. External senders will only be challenged once.
When the receiver's server get your mail (
user@yourdomain.com), it will send another mail to your SMTP server to check if that email (
user@yourdomain.com) is valid. If
user@yourdomain.com is not on that SMTP server, the receiver will reject your mail and your will get 550 5.7.1 Requested action not taken: message refused.