no such user but Thunderbird okeys it ?

Hi guys.

I'm having quite bizarre situation where Dovecot logs: ... pam_unix(dovecot:auth): check pass; user unknown pam_unix(dovecot:auth): authentication failure; logname= uid=0 euid=0 tty=dovecot ruser=dupa rhost=AA.BB.CC.DD imap-login: Login: user=<dupa>, method=PLAIN, rip=AA.BB.CC.DD, lip=AA.BB.CC.DD, mpid=1756629, TLS, session=<uV7OwIIEWsJdviSg>: ...

but Thunderbird allows, is okey with such user & creates an account for it. I must be having my setup miss-configured - I'm hoping it's something obvious somebody could point me towards.

many thanks, L.