Have you also troubles with user crontab and msmtp packages to be working together ?
configure well your msmtp to send email
echo "test yes" | mutt robert
it works, OK,
example, as user, edit crontab
0 8 * * * echo "test yes" | mutt robert
3/ wait and check your mail box
You too ?
I used :
nbsmtp and it is working
seems that there is BUGS somewhere