I am seeing the subjected error for a small percentage of messages, and then those message stay in the deferred queue.
from the log: postfix/local[1124]: warning: unexpected protocol delivery_request_protocol from private/bounce socket (expected: delivery_status_protocol) postfix/local[1124]: warning: read private/bounce socket: Application error postfix/local[1124]: A5EBC841EB: to=<redac...@redacted.com>, orig_to=<redacted>, relay=local, delay=25589, delays=25589/0.01/0/0.02, dsn=4.3.0, status=deferred (bounce or trace service failure) I have not found any hits searching on any of the following: private/bounce socket unexpected protocol delivery_request_protocol delivery_status_protocol So i have absolutely no clue what this error means, eg what is delivery_request_protocol and delivery_status_protocol?? Nor is there any pattern I can see with the small percentage of emails are involved. The only clue i have, is that this is only occurring on servers that were just upgraded from ubuntu 18 or 20, to ubuntu 22. Any leads would be greatly appreciated.
