Unfortunately, mail and mail-related services are being deprecated in macOS Server: Prepare for changes to macOS Server https://support.apple.com/en-us/HT20831

Unfortunately, mail and mail-related services are being deprecated in macOS Server: Prepare for changes to macOS Server https://support.apple.com/en-us/HT208312 https://support.apple.com/en-us/HT208312 The services deprecated include davmail, dovecot and postfix. They say the changes are "to focus more on management of computers, devices, and storage."