It has been a while since I touched ssmtp, so take what I’m saying with a grain of salt.
Problem with ssmtp and related when I was testing it was its behaviour in error conditions - due to a lack of any kind of spool it doesn’t fail very gracefully, and if the sending software doesn’t expect it and implement a spool itself (which it typically doesn’t have a reason to, as pretty much the only situation where something like sendmail would fail is a situation where it also wouldn’t be able to write a spool) this can very easily lead to loss of mails.
I already had a working SMTP client capable of fishing mails out of a Maildir at that point, so I ended up just doing a simple sendmail program throwing whatever it receives into a Maildir, and a cronjob to send this forward. This might be the most minimalistic setup for reliably sending out mail (and I’m using it an all my computers behind Emacs to do so) - but it is badly documented, so if you don’t care about reliability postfix might be a better choice, or if you don’t just go with ssmtp or similar. Or if you do want to dig into that message me, and I’ll help making things more user friendly.