Friday, February 7
Allowing Relaying from Certain Hosts
Sendmail doesn't like to relay mail that isn't sent from trusted sources. The designers of sendmail do this purposefully to try to alleviate the problem of spam. You see, spammers take advantage of mail servers that will relay mail from anyone in order to send mail to all of us while taking advantage of somebody else's bandwidth costs. It's truly heinous.
By default, sendmail's paranoia means that when we set up a server, we can only relay mail through it that originates on the local machine. In order to use it as a proper mail server, we need to let it know what hosts to trust to relay mail. For example, my mail sever is configured to accept email that comes from my private home network that is running behind a NAT with a fixed IP address. In addition, I always want to be able to send mail, using my laptop, from my friends houses which have known DSL hostnames. To do this, you simply need to define these rules in the /etc/mail/access file, as shown: