I'm interested in this as well, and having looked over the wiki2 pages on LDA and LMTP, and the files conf.d/15-lda.conf and conf.d/20-lmtp.conf to which they refer, I still don't see how the lmtpd knows a given user@domain is a system user. For virtual domains, I guess the assumption is that the Dovecot username is user@domain. (Even that assumption is not necessarily valid; there is no requirement to format virtual usernames that way.)
The closest I can find is "hostname" in 15-lda.conf, but that does not really say anything about it being used to identify a system user. -- http://rob0.nodns4.us/ -- system administration and consulting Offlist GMX mail is seen only if "/dev/rob0" is in the Subject:
