Welcome to the forums,
Just so you're aware NE version has an excellent add-on that does envelope forking called
Archiving & Discovery w/ cross mailbox search - often referred to as
ZAD for short.
(This can be used with a non-zimbra mailserver if you wish, though you'd loose the cross mailbox search abilities of course.)
More on it:
/pdf/Zimbra Archiving and Discovery Whitepaper.pdf /pdf/Zimbra Archiving and Discovery Release Notes.pdf
For FOSS many use the
always_bcc method of archiving.
If you're going to play with transport maps some recent thread are:
postfix transport maps &
Zimbra as filter/relay