[gentoo-user] Re: mail cannot send emails (trying to use it with smartd)

[Grant Edwards]

On 2020-04-03, Caveman Al Toraboran <toraboracaveman@××××××××××.com> wrote:

> though i'm a bit curious about sendmail (if your
> time allows).  do you mean the ebuild "sendmail"?

Yes. I meant the program provided by the "sendmail" ebuild.  That is
the MTA named "sendmail" that's been around since the universe cooled
enough to form atoms:

  https://en.wikipedia.org/wiki/Sendmail

For many years it was the de-facto standard MTA for Unix systems.

It's very powerful but the configuration file format is almost
impossible to understand, so people developed an m4 application that
accepted a _slightly_ less cryptic language and generated the sendmail
configuration file.  At it's peak back in the early 90's there were
approximately five people in the world who actually understood
sendmail, and none of them ever worked where you did.  The rest of us
stumbled in the dark using the finely honed cargo-cult practices
cutting and pasting random snippets out of example configurations to
see what happened.  Usually what happed is that mail was lost or flew
around in a loop multiplying to the point where a disk parition filled
up.

That said, sendmail has features that no other MTA has.  For example,
it can transfer mail using all sorts of different protocols that
nobody uses these days.

Back in the 90's a number of replacement MTAs were developed such as
qmail, postfix, exim, etc.  When you installed one of these, (instead
of the classic sendmail), they would usually provide an executable
file named "sendmail" that accepted the same command line arguments
and input format that the original did.  That allowed applications who
wanted to send email to remain ignorant about exactly what MTA was
installed.

Exim, postfix, qmail and the others were all still full-function MTAs
intended for a multi-users system. They could route mail to different
destinations (including delivering it locally to a variety of mailbox
types) and accept inbound email from other MTAs.  While they were far
easier to set up and maintain than the original sendmail, they were
still massive overkill for a computer that was used only by a single
person where reading mail was done via POP/IMAP and all outbound mail
was handed over to a single outside mail relay.  They often didn't
deal well with the fact that they were running on a host that didn't
have a "real" hostname that meant anything to the outside world, and
that the local hostname had nothing to do with the email addresses of
the user(s).

For that use case, simple MTAs like msmtp, ssmtp, and nullmailer were
written that don't handle incoming mail at all, and where all outbound
mail is sent to a single mail relay host.  The first two don't even do
any queuing: if you try to send mail when your relay host is
unreachable, then the send simply fails.

These too, when installed, provide an executable named "sendmail" that
accepts the same command line options and input format as the original.