this is a bug on the zimbra side.
manthrax3, I have filed bug 4933 to track this. Unfortunately, there appears to be no workaround (workaround would be a postfix config to normalize addr extensions before handing them off to MDAs, but I can't find one that would do that - which makes sense). Here is the text of the bug I filed:
postfix recipient_delimiter doesn't work with ZCS because (if
recipient_delimiter is, say, set to "+") postfix hands
"user+extension" to us and we are prepared to handle only
"user" and not "user+extension". So message gets bounced at
lmtp delivery stage.
The fix is to have a global LDAP attribute for recipient_delimiter,
propogate this to postfix main.cf and check in LMTP code if this
value is set and strip the delimiter and whatever follows it from
the address local part before attempting delivery.