Unix servers generally come with some kind of mail server installed. This is used to send mail generated by the system for log reports, error messages, etc… By default, it goes to the root account on the local server, but this requires a technician to log into each server and manually check mail. It is much better to have all mail forwarded to one or more e-mail accounts. See this article for instructions on how to set that up.
Long ago, in a galaxy far, far away, we could set up a central mail server to simply relay all e-mail from any random server. We would set up the root user on each server to actually be an e-mail account on a centralized machine, then check that one account. Then, the spammers got involved, started using these open relays to send out junk mail, and we had to turn them off.
Now, we can tell a centralized mail server to relay for some tightly controlled servers which we trust, but a better answer is to have relaying only done via authenticated connections. The remote machines actually send a username and password to the centralized (relay) server, then we can feel more confident the traffic is legitimate. This is used, for example, to get log and error messages from the satellite servers to the correct technician.
# mail host which handles our outgoing traffic for us relayhost = [smtp.example.com]:587 # credentials for relayhost are in this file smtp_sasl_password_maps = hash:/etc/postfix/sasl/sasl_passwd