So I've not been able to send email using my client on the desktop for about 8 months or so. Don't remember what changed or why, but at some point any attempt would just get rejected.
The other day, I finally dug into why, which meant putting on my virtual wading boots and going digging through the log files... (then into postfix configs. (🤮))
Every time I tried to send an email, it'd say Sender address rejected: not owned by user, because I was trying to send as tamber@[domain], logging in with the username tamber.
And what postfix was trying to do, apparently, was check the tamber@[domain] address and see what users were allowed to send mail using that address. Anyway, if you have reject_sender_login_mismatch set, and no previous rules that succeeded, it'll refuse if it can't find a matching username for that address. (Which has to be all set up in a virtual addresses file that's different to the other virtual addresses file I have, for whatever reason, I guess.)
Anyway, I just hammered permit_sasl_authenticated in there before the reject rule, and that's fixed it, 'cause it goes "yup, this user authenticated properly, good to go" before doing the convoluted lookup dance.
So when the topic comes up of like, running one's own email server because of the bullshit you get with the major providers, I would rather recommend against it unless you want this sort of shit to become your fucking job at the worst possible times and then you end up squinting at inscrutable config files at O-dark-30 in the morning. (And sometimes because you get a nastygram email from someone further up your network chain going "hey, your email server has been used to dump spam, fix it and/or die.")
