It was "listen" directive in dovecot.conf, nonetheless.
Replaced default "[::]" with "*" and everything worked. Apparently [::], while tuned to handle IPv6 interfaces, is less reliable for IPv4. My second mistake was trying to play with combinations of domain names and ip-s on that line: it doesn't like multiple entries.
Last edited by samoand; 6th January 2008 at 12:39.