if the suggestion from @luisBSBDATA (thanks for that!) doesn’t make email work, you might need to do more experiments, for example does “command line” mean a true shell or the web UI job Commandline option which would run as the same user as the scheduled job (might not be true for real command line)? You can also get the logging of email from the web UI job in several ways, for example MENU --> About --> Show log --> Live --> Profiling, or add the –log-file and –log-file-log-level=Profiling options. Here’s some output of my email sending:
2018-10-30 06:52:30 -04 - [Profiling-Timer.Finished-Duplicati.Library.Main.Controller-RunBackup]: Running Backup took 0:00:01:59.796
2018-10-30 06:52:34 -04 - [Profiling-Duplicati.Library.Modules.Builtin.SendMail-SendMailResult]: Whole SMTP communication: Connected to smtp://smtpauth.earthlink.net:587/?starttls=always
S: 220-elasmtp-mealy.atl.sa.earthlink.net ESMTP Exim 4 #1 Tue, 30 Oct 2018 06:52:33 -0400
<setup then email>
C: QUIT
S: 221 elasmtp-mealy.atl.sa.earthlink.net closing connection
2018-10-30 06:52:34 -04 - [Information-Duplicati.Library.Modules.Builtin.SendMail-SendMailComplete]: Email sent successfully using server: smtp://smtpauth.earthlink.net:587/?starttls=always
2018-10-30 06:52:34 -04 - [Information-Duplicati.Library.Main.Controller-CompletedOperation]: The operation Backup has completed
If you see something then maybe it will give clues to what went wrong. If you see nothing, that’s a clue too. 