There are many instances where one may only want to send mail through a relay. Some examples are:

While any MTA is capable of filling this particular niche, it can be difficult to properly configure a full-featured MTA just to handle offloading mail. Programs such as Sendmail and Postfix are overkill for this use.

Additionally, a typical Internet access service agreement may forbid one from running a “mail server”.

The easiest way to fulfill those needs is to install the mail/ssmtp port:

# cd /usr/ports/mail/ssmtp
# make install replace clean

Once installed, mail/ssmtp can be configured with /usr/local/etc/ssmtp/ssmtp.conf:

root=yourrealemail@example.com
mailhub=mail.example.com
rewriteDomain=example.com
hostname=_HOSTNAME_

Use the real email address for root. Enter the ISP's outgoing mail relay in place of mail.example.com. Some ISPs call this the “outgoing mail server” or “SMTP server”.

Make sure to disable Sendmail, including the outgoing mail service. See 節 28.4.1, “關閉 Sendmail” for details.

mail/ssmtp has some other options available. Refer to the examples in /usr/local/etc/ssmtp or the manual page of ssmtp for more information.

Setting up ssmtp in this manner allows any software on the computer that needs to send mail to function properly, while not violating the ISP's usage policy or allowing the computer to be hijacked for spamming.