Debian – How to Customize Email Headers from Vixie-cron and msmtp

I have had some trouble with getting cron to send emails. My ISP requires that the "From:" field match the email address the email is being sent from, otherwise the email is rejected. As cron hardcodes the "From:" field to "root (Cron Daemon)", these emails are not being sent.

I have set up msmtp and can send emails with no problem from the command line. In fact I have wrapped some of my cron jobs in a script that sends the email itself. This works fine, but I would like a more elegant solution.

I originally used the package msmtp-mta, which just symlinks /usr/lib/sendmail to /usr/bin/msmtp so that cron will send emails using msmtp. Since that didn't work, I removed the package and put a bash script in /usr/lib/sendmail instead that should just read from standard input and send an email with the right headers:

#!/bin/bash

HEADERS="To: <myemail>
From: Cron <myotheremail>
Subject: Vixie-cron snooper ($@)

"

INPUT=$( cat /dev/stdin )

echo -e "$HEADERS""Stdin:\n$INPUT\n" | msmtp <myemail>
echo "$HEADERS""Stdin:\n$INPUT\n" > /tmp/vixielog

However, this doesn't have the desired effect. I just receive an almost empty email and /tmp/vixielog contains the same:

To: <myemail>
From: Cron <myotheremail>
Subject: Vixie-cron snooper (-i -FCronDaemon -oem <myemail>)

Stdin:

The emails come at the right time, so I know the cron job is being run properly, but I am not getting the output. How could I adjust this approach to get the output of the command in the email?