I don't have an IPv6 PTR and I send mails through a .xyz domain and everything still works at least when sending to a gmail inbox.

I managed to set up an SMTP server on my instance, I think I used postfix and dovecot after following some random tutorial. Then followed it with some generic make your mail not fall into spam tutorial to have the DKIM records and whatnot and to my surprise the e-mails reached even my Gmail inbox, which has probably the most strict rules. I must say that setting up postfix with dovecot is the hardest part, at least for me, after that it was just copy-pasting commands and setting some DNS records.

FairEmail is a great email client. Also everytime I reinstall my phone, I get the SimpleMobileTools line of apps, their apps like gallery or calendar are nice-looking and useful.