I can't repro that in Xenial. The error message I get comes from the ssmtp sendmail: cannot open mail:25
If you can run strace mpack path/to/file <email address hidden> we could at least see what's wrong.
Also, make sure your PATH is correct. Out of the box on Ubuntu, ssmtp installs /usr/sbin/sendmail which might not be in your PATH if you are running as a mortal user.
I can't repro that in Xenial. The error message I get comes from the ssmtp sendmail: cannot open mail:25
If you can run strace mpack path/to/file <email address hidden> we could at least see what's wrong.
Also, make sure your PATH is correct. Out of the box on Ubuntu, ssmtp installs /usr/sbin/sendmail which might not be in your PATH if you are running as a mortal user.