Mail only works on localhost

This happens quite often with new users or admins who are not familiar with mail setups. Postfix (/etc/postfix/main.cf

inet_interfaces = localhost

Change to

inet_interfaces = all

Sendmail (/etc/mail/sendmail.mc

DAEMON_OPTIONS('Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

Change to

DAEMON_OPTIONS('Port=smtp,Addr=0.0.0.0, Name=MTA')dnl

Services need to be notified of the changes

sendmail service restart

or

service postfix reload