smtprelay VS django-sendmail-backend

If you have an always on host within the office and accessible to the copiers, even a RPi, there are tons of solutions on Github (ex. https://github.com/decke/smtprelay). A Windows machine I will typically configure hMailServer with MariaDB. For all-cloud, SMTP2Go is really the only solid choice I have found. Others I tried were either more pricey or required a super long key for authentication that copiers did not like.

Yep. I use https://github.com/decke/smtprelay for this purpose.

I use smtprelay, an alternative to the pain in the butt postfix, 3 lines and you are ready to use it to receive email, anything sent to it is automatically "relayed" to your email provider (127.0.0.1) via STARTTLS.

I know about dma, ssmtp, msmtp, nullmailer and https://github.com/decke/smtprelay/, but these don't seem to support queuing and retries.

This is what I use: https://github.com/decke/smtprelay

As I'm learning more I see I didn't formulate my requirements quite right. My client app is a SMTP client (a Django webapp, it uses Django's default email backend, which uses python's smtplib). It needs a TCP port to connect to, but looks like both nullmailer and dma doesn't offer that. Instead you have to inject messages in the queue by calling the sendmail binary. Now, there's a 3rd-party Django email backend that uses sendmail but that has its own new set of problems ;-)