I would like to use sendmail in a docker container to send email. I was able to do this before for my own site when I used a certain script to set everything up.
When I try this now sendmail tells me Deferred: Temporary AUTH failure but the next line says Message accepted for delivery. However, the message is not sent as I use Mailgun for SMTP credentials and their logs never report the email that sendmail tries to send. I have confirmed my Mailgun credentials are still valid because I can use their REST API to send mail.
I noticed when I change the port from 587 to 465 or 25 in the sendmail.mc, it is not able to connect to smtp.mailgun.org at all. It is only when I use port 587 that I get as far as the Temporary AUTH failure. How can I resolve this authorization failure? I'm 100% sure that the correct credentials are in my /etc/mail/authinfo/auth file. I tried ending that file with "M:LOGIN", "M:PLAIN", and "M:PLAIN LOGIN", and "M:EXTERNAL" but to no avail.
How on earth do I configure sendmail to send mail with my Mailgun SMTP credentials over port 587 without this "Deferred: Temporary AUTH failure" error?