Many to One Mail-User Forwarding

I have two mail-user accounts that need to forward to the same external recipient. How can this be done?

The question seems simple enough, but when we try this, errors are reported:


[The proxy address “” is already being used by “demolab.local/Users/UserA”.  Please choose another proxy address]

As you can see, the issue isn’t actually with the ExternalEmailAddress attribute (technically called targetAddress within Active Directory) we are trying to set; it’s the proxy address.

When UserA was created, Exchange created a proxy address to match the ExternalEmailAddress. When we try to create UserB, Exchange also tries to create a proxy address that matches the ExternalEmailAddress. And in this case, because was already assigned as a proxy address to UserA, UserB could not be created.

Initially I thought: I’ll create a contact and have both these mail-users forward to that contact. But this doesn’t work. First of all, now we would have to create a 3rd directory object which is undesirable, but more importantly that “forward to a contact” feature I was thinking of only applies to mailboxes (Set-Mailbox –ForwardingAddress).

Then I thought: Well, just make `em mailboxes! This would work, but there are too many associated risks. What if the user were to find out about their mailbox and login and start using it? The “-DeliverToMailboxAndForward $false” would prevent it from collecting mail, but not if the user starts filling Sent Items or other mail folders. I even thought of disabling these user accounts, but users need the accounts for other non-Exchange functions, so mailboxes are out.

This is a stupid problem to have. I understand multiple objects with the same proxy addresses could result in inconsistent mail delivery, but why does the proxy address have to match the ExternalEmailAddress in the first place!?

The closest thing I could find to this was here: Proxy-Sync Scenarios. Within you’ll see that not only does this specifically speak to older versions of Exchange (a lot has changed with attributes in Exchange 2007/10), but the author is talking about mailboxes, not mail-users.

So what happens if we take things into our own hands? Can we just remove the unwanted proxy address from UserA so that I can create UserB? In the GUI the answer seems to be NO. The red “X” that deletes proxy addresses greys out when you select a proxy address that is used by the ExternalEmailAddress field:


But what happens if we try to write over the value in the Exchange Management Console with a list which does not include the ExternalEmailAddress?

Set-MailUser UserA –EmailAddresses “UserA@demolab.local”


This works! Let’s verify:


Now we can go back and create UserB with the same external email address as UserA.

Two closing thoughts:

1. To be a tidy admin, I’d recommend removing the superfluous address from UserB as well. Better to be consistent.

2. The intentional graying out of the delete button in the EMC makes me wonder if Microsoft simply forgot to include the same safeguards in the shell. I cannot foresee problems with this approach, but it makes me wonder why the attributes for mail-users are linked in the first place. Follow my guidance here at your own risk.

3. I realize you could also approach this problem by using ADSI edit or some other manual approach, but I believe you have a higher level of “supportability” from Microsoft when you use the intended tools for the job. As I noted in #2 above, this may not be supported, but at least we tried!


Thanks Will for your help with this one!

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