Occasionally we get reports from customers that they have deleted a mailbox on Exchange and set up an alias for the old address to another mailbox, but the forwarding alias doesn't work and message sending fails.
Here's an example:
John works at a company using Hosted Exchange. He leaves and the company want to ensure that any messages sent to him are still picked up, but they don't want to have to log in to his mailbox to do this.
So they delete his original mailbox and set up an additional email address on Helen's mailbox.
When a test email is sent to John's old address, everyone expects it to appear in Helen's mailbox, but instead an error is received that states:
"Delivery has failed to these recipients or groups"
In most cases the issue here isn't with the Exchange service, but actually within Microsoft Outlook. When you send an email to someone within your Exchange organisation, Outlook automatically stores the unique mailbox reference in its autocomplete database. The next time you send an email to that address, it simply uses the mailbox reference that it has stored.
This usually works fine, however in our example above we have deleted that mailbox. Therefore you need to remove the stored entry from Outlook. The easiest way to do this is to open up a new email message and start typing the name of the old mailbox. When it pops up highlight it with the mouse and without clicking it, select the delete icon alongside it.
This will remove the autocomplete entry. Now simply resend a test message to the email address to verify that it's now working.
Please note that this issue will only affect any users within your Exchange organisation that use Outlook. Any messages sent to the email address from outside your company will be routed successfully.