Sendmail

cat << EOF | sendmail -t 
Sender: [email protected]
From: Sender <[email protected]>
Reply-To: Sender <[email protected]>
To: To <[email protected]>
CC: Cc <[email protected]>
Disposition-Notification-To: [email protected]
Subject: Generic Subject

Generic Body
EOF

Knowledge

/usr/sbin/sendmail -t < filename.eml
/usr/sbin/sendmail -v -t < filename.eml
/usr/sbin/sendmail -Am -v -t < filename.eml

env command is to launch interpreters
1. Python 3.9: #!/usr/bin/env python3.9
2. Python 2.6: #!/usr/bin/env python2.6
3. Python    : #!/usr/bin/env python
4. Perl      : #!/usr/bin/env perl

References

Configuring SendMail to use username and password Configure sendmail for relaying mail over port 587 Capture debug output from Python smtplib Setup and test SMTP AUTH within Sendmail Sending mail from a Bash shell script Sending a mail from a shell script Reply-To header to send emails Sendmail Attachment Python Send Email