Occasional 'Protocol Errors' - Cyrus 2.4

[Karl Pielorz]

Hi,

I've noticed in my logs - very occasionally Cyrus isn't excepting 
messages...

The front end MX host accepts the mail, passes it onto the back end IMAP 
server - but this logs:

sm-mta[41247]: yyyyyyyyyyy: localhost: SMTP DATA-3 protocol error: 530 
5.3.0 <auser> Fatal: 550-Mailbox unknown.  Either there is no mailbox 
associated with this

While trying to deliver it to the imap inbox (via LMTP etc.)

I've also seen:

sm-mta[70586]: zzzzzzzzzz: localhost: SMTP DATA-3 protocol error: 530 5.3.0 
<buser> Fatal: 501 5.5.4 Syntax error in parameters


Any idea how I can try and debug this? - From / To Address combinations 
that deliver fine, will then randomly fail at some future point.

Checking this month the server has rejected 4 out of over 10,000 accepted 
messages. Annoyingly they're a hard 5xx fail (so it goes back to the 
sender).

This is on FreeBSD 10.3.


-Karl