Switched to using dovecot for POP3, PLAIN login.
One client with "Ant Internet Suite" can't use the service with this client, he used the same setting with another email client and it worked.
On inspection of the packets, it is sending one packet with;
"user username\r\npass password\r\nstat\r\n"
Dovecot sends an "-ERR Unknown command", then a "+OK Logged in".
I'm wondering if things got out of sequence here, since a "STAT" to dovecot before authentication causes the "Unknown command" error. "STAT" isn't allowed in the authentication stage... but well it issued it last....
Anyway I'm unclear here on the precise details of the packet trace, possible it is responding to an earlier packet, but thought I'd ask before pursuing this further in case some one leaps up and says "oh that'll be...".
The client software has an option to disabled "queued commands" buried deep within it (append /ucb to user details), I suggested trying this if possible, will report back if it works. Also mentioned in a Google search.
I'm curious as to what is right here, but the "/ucb" is presumably the practical answer if it works.
Simon