- Make sure server host / ISP doesn't block ports. - Request IP to be removed from blacklists if it is on some due to IP having previously been abused. - Request some mail providers specifically whitelist it because they have their own whitelists/blacklists. - Make sure to implement SPF, DMARC, and DKIM. - Make sure you don't accidentally set it up as an open relay. - Make sure PTR records are set correctly for rDNS.
I may have forgotten some steps lol. The FeministWiki has a mail server that seems to be working quite well, but still occasionally runs into issues.