Dovecot

IPTables Ports
If in doubt check /etc/services

pop3 110/tcp      
pop3 110/udp
imap 143/tcp
imap 143/udp

pop3s 995/tcp      
pop3s 995/udp
imaps 993/tcp
imaps 993/udp

Lock dovecot down to specific users
# vim  /etc/pam.d/dovecot
#%PAM-1.0
auth       required     pam_nologin.so
auth       required     pam_listfile.so item=user sense=allow file=/etc/dovecot_users onerr=fail
auth       include      system-auth
account    include      system-auth
session    include      system-auth

Testing dovecot
mutt -f imaps://user@server