That is the exact problem that got me to abandon my splendidly working, self-hosted email stack. I had so many instances of mail just not getting delivered, despite doing absolutely everything correctly, that I gave up.

Thankfully not a problem we've encountered yet. All my mail is DKIM signed, SPF aligned etc. I also registered on dnswl.org which is a signal quite a few people seem to use.

I’m wondering what the most fool proof way to get a message delivered to someone else is. I found out today some SMS messages I sent were silently dropped. Email doesn’t always get delivered. I would hope the closed ecosystems like Signal and Whatsapp are better at this and in any case you need both parties to agree on a channel. I just like email because my identity is maintained even when I switch providers, and nothing else quite like that exists.