How does mailutils "mail" command or cron contact local MTA?

[Yvan Masson]

Hi,

I have some servers successfully configured to send notification emails,
but I do not exactly understand how it works :-)

I have installed openSMTPd and configured it to forward email to a relay
(another SMTP server, the one from my email provider). I have also set
up aliases in `/etc/aliases`. This allows me to send email from my
servers, either when cron runs a task, or when I run `echo my_content |
mail -s subject root` from scripts.

My question is: how cron daemon and mail command know that they should
contact openSMTPd, and how? Do they try by default on localhost:25 or
/run/smtpd.sock? I could not find any clue, nor in cron manpages nor in
mailutils online documentation.

Regards,
Yvan

[Greg Wooledge]

On Sat, Sep 17, 2022 at 11:11:51PM +0200, Yvan Masson wrote:
> My question is: how cron daemon and mail command know that they should
> contact openSMTPd, and how? Do they try by default on localhost:25 or
> /run/smtpd.sock? I could not find any clue, nor in cron manpages nor in
> mailutils online documentation.

mail(1) or mailx(1) or similar utilities run the /usr/sbin/sendmail
program to inject a new message into the local MTA.

/usr/sbin/sendmail was of course originally provided by the sendmail
package. But later MTAs implement a /usr/sbin/sendmail compatible wrapper
so that all the utilities that send mail don't have to change.

[Yvan Masson]

Thanks Greg!

Indeed, just to confirm for other interested on this topic, this is
documented for `mail` command on
https://mailutils.org/manual/mailutils.html#Mail-Variables, and for
Vixie/ISC cron (default cron daemon on Debian) on
https://salsa.debian.org/debian/cron/-/blob/master/config.h.

So the `mailutils` package with `mail` command is optional: `sendmail`
could be used directly in scripts (but it might be a bit less practical
to use, see for example
https://www.computerhope.com/unix/usendmai.htm#examples).

Regards,
Yvan

[Greg Wooledge]

On Sun, Sep 18, 2022 at 09:27:46AM +0200, Yvan Masson wrote:
> So the `mailutils` package with `mail` command is optional: `sendmail` could
> be used directly in scripts (but it might be a bit less practical to use,
> see for example https://www.computerhope.com/unix/usendmai.htm#examples).

If you call /usr/sbin/sendmail, you have to write all of the headers
yourself, with correct (or at least acceptably close) syntax.

The only time a shell script should be calling /usr/sbin/sendmail directly
is when you're trying to do something that your MUA (mailx, et al.) doesn't
allow, such as forging a bogus Date: header, or overriding the envelope
sender address.

In normal usage, scripts should always be calling mailx or an equivalent
program which is *designed* to create the e-mail headers correctly. This
is especially true if you're sending attachments. Writing the MIME headers
and attachment markup yourself is not going to be fun.

[Teemu Likonen]

* 2022-09-18 09:00:49-0400, Greg Wooledge wrote:

> In normal usage, scripts should always be calling mailx or an
> equivalent program which is *designed* to create the e-mail headers
> correctly. This is especially true if you're sending attachments.
> Writing the MIME headers and attachment markup yourself is not going
> to be fun.

Mailx, or even better, Mutt:

mutt [-nx] [-e command] [-F rcfile] [-b bcc-addr] [-c cc-addr]
[-s subject] [-a file ... --] to-addr ... < message

--
/// Teemu Likonen - .-.. https://www.iki.fi/tlikonen/
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