After resetting networking on my iPhone, I ran into LetsEncrypt cert issues on my phone again (with my mail server). For some reason iOS keeps trying to use an old cached version of the certificate.
Finally found the solution. And all it was, was to configure Dovecot and OpenSMTPd to use the .fullchain.pem instead of the .crt for that domain.
Everything is happy again.
A bunch of technomancers in the fediverse. Keep it fairly clean please. This arcology is for all who wash up upon it's digital shore.