[Dovecot] Postfix/Dovecot/lmtp with virtual and local users

[/dev/rob0]

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 at domain is a system user. For virtual 
domains, I guess the assumption is that the Dovecot username is 
user at 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.
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