You could try mail extentions, whereby an address of
user@domain.com can become
user-ZimbraForum@domain.com and
user-randomwebsite@domain.com
The mail will be delivered to the mailbox addressed before the - and the rest of the address is basically ignored.
At some point in the future, I hear that zimbra will use the suffix after the - to file the mail in a folder by that name if it exists, so that may help. Currently you cannot select the outbound email address, but I hear that may change too.
This is not an exact solution to your request, but it's something to think on.