Unfortunately most of that is up to the receiving end of an email. Not much you can do. If they don't get your email, run a dns check on there mail server and see if it's blocking your mail server ip address. If so , they will need to contact there mail hosting provider and unblock it. I used to do...

there are some good threads over on hn: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&query=mail%20server&sort=byPopularity&type=story

I ran my own for years; stopped shortly after Gmail was first released. Sad that it’s no longer generally feasible, and I appreciate that I was part of the problem.

Don't use tor. Lol

You could run multiple mx records if you wanted, prob don't need to tho. You'll need to make sure you have proper dns ptr record for it. I'd recommend postfix. The time consuming stuff is giving users access w imap and etc, managing users, mail formats.

Do you have DKIM and SPF configured on the domain? I don’t self-host, but I do a lot of troubleshooting and those two are often the culprit when it comes to deliverability.

https://blog.lopp.net/death-of-decentralized-email/amp/

Who remembers when people utilized Pop protocol for incoming mail. Such a disaster.

I self host a mail server. Its not hard. Make sure dkim, spf, and dmarc is set properly or you will have issues. I went as far as setting up DNSSEC as well. If nothing else its good experince.

I used to run a mail server for myself and all of my clients. I couldn't imagine doing this today. It's such a daunting task.