The main problem I've run into while running my own email server is IP reputation issues. It is still an ongoing issue for me. You can read my previous IP reputation issue in this comment [11].

You should make sure your server's IP address isn't blacklisted. If it is, you're going to have major delivery issues with some email server providers (ESP). Some blacklists you can check are listed here [0]-[3].

I think my main problem now is the UCE Protect [5] blacklist. I think some of the major ESPs use their harshest blacklist from UCE Protect, which is their Level 3 blacklist [6]. This blacklist will include your IP if your ISP meets a spam threshold for any of their other IP addresses. This makes running a mail server on cheap hosting providers like Digital Ocean or Linode very difficult.

My conclusion is I should switch to a more expensive ISP that isn't in danger of getting on the UCEPROTECTL3 list or find an email forwarding service for a next hop destination for outgoing mail.

You can read more about UCE Protect here [7]-[10].

[0]: mailtester: https://www.mail-tester.com/

[1]: mxtoolbox blacklists: https://mxtoolbox.com/blacklists.aspx

[2]: proofpoint blacklist: https://ipcheck.proofpoint.com

[4]: outlook blacklist: https://sendersupport.olc.protection.outlook.com/snds/index....

[5]: UCEPROTECT: http://www.uceprotect.net/en/index.php

[6]: UCEPROTECTL3 blacklist: http://www.uceprotect.net/en/index.php?m=3&s=5

[7]: UCEPROTECT Blacklist Scam https://community.spiceworks.com/topic/2170592-uceprotect-bl...

[8]: UCEPROTECT: When RBLs Go Bad https://blog.sucuri.net/2021/02/uceprotect-when-rbls-go-bad....

[9]: ASK HN thread https://news.ycombinator.com/item?id=26064722

[10]: SQLite3 IP blacklisted: https://sqlite.org/forum/forumpost/bb61881d7a?hist

[11]: Previous IP reputation issue https://news.ycombinator.com/item?id=25437841