what should it do with the non local messages or local messages directly at aliases?
OK, so I gather that the Submission Server cannot do that (yet).
My suggestion for a future version would then be: How about running dovecot-lda, if the user happens to be local, or a local alias? Or at least provide some sort of pattern matching: anything matching *@example.com , pass the message to dovecot-lda .
It feels strange that a plug-in accessing the local user database for authentication purposes, and running on the same Dovecot server instance, needs to use an MTA to deliver a local message, it is like going out to come back in again. But I do not know much about mail servers yet. Have I missed some important concept here that makes this idea silly indeed?
Regards, rdiez