If you cannot connect remotely to sendmail, postfix or other mail servers on port 25 make sure you have the port opened on your firewall. This article deals with opening port 25 with iptables on RedHat Linux.
How to enable sendmail to accept remote connections. By default most distributions (or when compiling sendmail yourself) will only allow local connections for security reasons.
When you restart dovecot, if it won’t start up again it’s likely the process crashed on startup but left the subsys lockfile in place. This prevents dovecot from starting up again. If you run
/etc/init.d/dovecot status and get the error message "dovecot dead but subsys locked" then you have this problem.
If you get a message like the one following, then you are filtering emails through spamassasin using /usr/bin/spamc but the spamassassin service is not running. connect(AF_INET) to spamd at 127.0.0.1 failed, retrying (#1 of 3): Connection refused.
I have been setting up a new mail server recently with Postfix and SMTP Auth, and got the error message "no SASL authentication mechanisms".