One option would be to remove the SMTP address from the account prior to deletion. Another option would be to adjust your deleted mailbox retention period.
To confirm that this is what is happening, launch your Exchange Management Console and navigate to the mailbox store that contained the deleted mailbox. Navigate to the Mailboxes object. You should see the phantom mailbox with a red X beside it. If not, right-click Mailboxes and then click Cleanup Agent, it will now show up with a red X.
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