I have my main desktop "dev2", and a newer "hot backup" machine named
"dev3". A "singing, dancing" cron job runs daily at 03:30 local time.
root's crontab is...

30 03 * * * /root/bin/bkup

The script "/root/bin/bkup"...
* uses "wakeonlan" to fire up my "hot backup" machine
* waits 30 seconds for it to boot up (NVME is nice!)
* rsync my $HOME directory, and a VM image in another directory, to
the "hot backup" (exclude .ssh directory and some other stuff)
* ssh root@dev3 /sbin/poweroff

The script works, but the cron job email output has problems. My ISP
is "EBOX" https://www.ebox.ca/en/ontario/residential/ My email to this
list ccomes from "mutt" MUA which is set to use a 3rd-party service
(cotse.net). Cron jobs come from <wherever?> and default to using ssmtp
on my machine, which is configured to use EBOX mail server as follows...

#
# The person who gets all mail for userids < 10
root=walterdnes
# The place where the mail goes. The actual machine name is required
# no MX records are consulted. Commonly mailhosts are named mail.domain.com
# The example will fit if you are in domain.com and you mailhub is so named.
mailhub=smtp.ebox.net
# Where will the mail seem to come from?
#rewriteDomain=waltdnes.org
# The full hostname
hostname=waltdnes.org
FromLineOverride=YES

This seems like a "Rube Goldberg contraption"
https://en.wikipedia.org/wiki/Rube_Goldberg_machine where...

* the cron job sends an email to my ISP's smtp server
* which sends it to my inbound personal domain email host cotse.net
* which I poll manually occasionally

Is this *REALLY* necessary? Can I turn off cron job emails alltogether?
The failure seems to happen when the EBOX server submits the cron job
email to cotse.net. Which one of them is "out of spec"? Here is the
relavent part of the diagnostics...

===========================================================================
This is the Email Security Gateway at host barracuda.ebox.ca.

I'm sorry to inform you that the message below could not be delivered.
When delivery was attempted, the following error was returned.

<walterdnes@???>: host mail.cotse.net[173.166.91.11] said: 421 4.5.0
    Bare carriage return (CR) not allowed (in reply to end of DATA command)
===========================================================================

--
Walter Dnes <waltdnes@???>
There are 2 types of people in this world
1) Those who can extrapolate from incomplete data