What this means in practice is that you have to set up with DKIM and SPF at minimum. E.g.: https://www.exratione.com/2016/05/a-mailserver-on-ubuntu-16-04-postf

What this means in practice is that you have to set up with DKIM and SPF at minimum. E.g.: https://www.exratione.com/2016/05/a-mailserver-on-ubuntu-16-04-postfix-dovecot-mysql/ https://www.exratione.com/2016/05/a-mailserver-on-ubuntu-16-... But: you must then also use a host that is going to give you a decent chance at a non-polluted IP address. The host of the server appears to be a big factor in the hidden part of the anti-spam ecosystem used by the big email providers like Gmail and Hotmail. This means that trial and error and experience in the ever-changing field is the only way to make a good choice. Expect to be moving around a bit as you figure things out. AWS EC2 is the best choice that I know of. Digital Ocean is the worse choice that I know of. A possible approach is to set up your mailserver so that it relays outbound messages via AWS SES. SES has all of the advantages of a big service that puts in time to ensure deliverability, with few of the downsides, such as large cost. For calendar apps, you might look at Horde (see an old setup example here https://www.exratione.com/2012/05/a-mailserver-on-ubuntu-1204-postfix-dovecot-mysql/ https://www.exratione.com/2012/05/a-mailserver-on-ubuntu-120...) - I don't know how it matches up with other options. I moved away from it because I really didn't need a calendar as it turned out, and Horde is something like 10% application and 90% configuration system. It was a lot of work to get it set up and comfortable.