Now that Apple has abandoned mail server capabilities in macOS Server.app we have to move to an open source solution. I have done this using Macports and it works well (no thanks to Apple as it never published the in 2018 promised migration info for mail services).
The MacPorts Apple Push Notification variant based on a patch written a few years ago for dovecot supports APNS but it requires you to export a certificate form your old High Sierra Server.app. With the patch and that certificate, dovecot tells an email client it supports XAPPLEPUSHSERVICE and that makes Apple mail clients think they are talking to macOS Server and they switch to push notifications for updates. Now, that certificate needs to be renewed yearly by Server.app and imported by hand etc. At some point this will stop working (apart from that you need to keep an old macOS Server around to renew)
Is there another way to get this working with an open source dovecot, outside of using Apple’s push notification service? Some other standard? Some other service? Probably not, right?
G