Thank you, It worked!
conf.php was under /etc/psa-webmail/horde/horde (also under /usr/share/psa-horde/config)
It had option "if safemode=1 then smtp else sendmail" i commented out "sendmail" and "if" and leaved only smtp with auth=true (because i disabled localhost from trusted list. spammers were hacking clients web sites and were sending tons of spams via script from localhost)
Unfortunatly i couldn't find conf file for atmail to use smtp, so i just disabled it.