For generating Message-Id header ALPS uses github.com/emersion/go-message/mail module. That one generates unique ID and appends machine hostname. While this is correct technically I am thinking of two little suggestions why I'd like to change this behaviour:
go-message/mail uses recommendations from the RFC to generate Message-Ids. The problem with using the From domain is that it's harder to guarantee we won't run into a Message-Id conflict.
I know it's better to use host name of the machine generating the message. It's also good idea to make the message ID end in an FQDN. Sender domain is easier to retrieve than FQDN host name thus I have proposed it. At the end I would just prefer if message-id would contain FQDN and I don't really care much what domain that is.