======  Is it possible to have global virtual aliases for addresses like postmaster, so that mail for postmaster at all of my domains can go to the same address? ======

In the past the answer was no. However, with current versions of Postfix, you can create regular expression maps and include a wildcard when an address at all domains should go to a single mailbox. For example, if you have several virtual alias domains, you can create entries like the following in main.cf:

virtual_alias_domains = example.com, oreilly.com, ora.com
virtual_alias_maps = regexp:/etc/postfix/virtual_alias

And in /etc/postfix/virtual_alias include an entry like this:

/postmaster@.*/            admin@seaglass.com

Then all mail for postmaster@example.com, postmaster@oreilly.com, and postmaster@ora.com will go to admin@seaglass.com.

If any of your virtual alias domains requires a different alias mapping, then this shortcut won't work. You will need a separate entry for each domain. With a script or makefile this is a very simple thing to maintain.

source: http://www.seaglass.com/postfix/faq.html#vrtglal 

====== config links ======
  * https://help.ubuntu.com/community/DovecotLDAP
  * http://wiki2.dovecot.org/AuthDatabase/LDAP/Userdb
  * http://wiki.dovecot.org/VirtualUsers/Home