dovecot - Oct 2011 - Only using LMTP-delivery, where should domain & user aliases be configured, Dovecot or MTA conf?

I have my MTA (Postfix) check Dovecot2 LMTP for existing users.  No
other delivery is done; this is a virtual/LMTP-only setup.

I define known users in a Dovecot passwd-file, specified in

conf.d/10-auth.conf
 passdb {
  args = ... username_format=%n /stor01/vmail/auth.d/%d/passwd

e.g.,

 vi /stor01/vmail/auth.d/mx1.domain1.com/passwd
  myuser:{scheme}####:...

mail to: myuser at mx1.domain1.com is correctly auth'd and stored.



I want two things to happen:

(1) all mail to existing-users "@domain1.com" gets re-routed to
"@mx1.domain1.com", i.e.,

    map:  myuser at domain1.com -> myuser at mx1.domain1.com

and is accepted/not-rejected ONLY if 'myuser' is defined as a user in
mx1.domain1.com/passwd

and,

(2) specific 'user' aliasing/remapping within a given domain, i.e.

    map:  whatever at mx1.domain1.com -> myuser at mx1.domain1.com

as long as 'myuser' is a defined user in mx1.domain1.com


In these cases, where should I assign domain & mail aliases, Dovecot or
the MTA's conf ?