To allow backuppc user to send emails I've to change permissions to /etc/msmtprc file:
chmod a+r /etc/msmtprc
Now I can send email as backuppc, but there is a problem with logging:
root@NAS:~# sudo -u backuppc /usr/sbin/sendmail jaqb@mydomain.com
subject:test
from:backuppc@nas
this is a test
sendmail: cannot log to /var/log/frontview/msmtp.log: \
cannot open: Permission denied
sendmail: log info was: host=mail.mydomain.com tls=on auth=on \
user=jaqb from=jaqb@mydomain.com \
recipients=jaqb@mydomain.com mailsize=90 smtpstatus=250 \
smtpmsg='250 2.0.0 Ok: queued as E29FC1665F31' exitcode=EX_OK
(after "this is a test") you have to press Ctrl-D (end of file).
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