The most important thing is to setup DKIM, SPF and DMARC correctly. I mean sign the emails with the correct key, but also setup the correct records, e.g. SPF re

The most important thing is to setup DKIM, SPF and DMARC correctly. I mean sign the emails with the correct key, but also setup the correct records, e.g. SPF record must be TXT, if you create a (deprecated) SPF type record GMail won't recognize it, even if you also have the TXT one! Of course they don't mention it in the documentation, and I've spent 1 month of trial and error to figure it out. And the DKIM record needs to be formatted exactly correctly (antother thing that took me time to figure it out) With DMARC you can specify that the recipient sends you a periodc report of the delivery status, this is useful to debug eventual problems. For example GMail and Outlook send this out, and gives you information about the fact that your DMARC is working correctly and SPF and DKIM checks are passed. Anyway when you manage to get it working, it did work for around 4 years without any issues. I've even reinstalled the mailserver to another machine (keeping the same IP) and I did not have any issue in the migration.