I'll try to fill in. I'm very aware that I'm in a bubble with my peers, so these are certainly opinionated. I would pick Postfix because it's more maintained,

I'll try to fill in. I'm very aware that I'm in a bubble with my peers, so these are certainly opinionated. I would pick Postfix because it's more maintained, and what most of my peers use it too. It's a PitA to configure, but there seems to be many resources online. Most of the configuration are quite common, so that one-time configuration is manageable. Exim is pretty robust too. I don't have a strong reason to not use it, except that I don't have many close peers who use it. Really nice configuration. If you have a multi-tenant setup, Exim would serve better. Qmail, despite being unmaintained, is praised for being quite secure. I suppose it's the low number of CVEs. There is also msmtp that provides sendmail compatibility, but uses an external smtp server (like AWS SES) to send the actual email. This is what I use when the server I'm sending from has low IP reputation but I don't want to change my app to use a REST API to send email.