Solution: Proxy address conflict when adding an email address in Exchange Online

This proxy address conflict, duplicate email address, email assigned to multiple Exchange object’s proxyaddresses or EmailAddresses is the issue plaguing me for a while. It affected in assigning Microsoft 365 licenses by group-based license, adding/removing mailboxes to a distribution group, and other places.

Usually the error message says it can’t process an email address because it is assigned to multiple mailboxes (or other Exchange objects). You might have end up at https://learn.microsoft.com/en-us/troubleshoot/exchange/email-alias/proxy-address-being-used. This site says run a Get-Recipient -Filter {EmailAddresses -like “*Conflicting-Email-Address*”}. In every situation, it returned only one Exchange object (mailbox or contact or DL). I never found a two or more object had that email address in my environment.

Solution:

I finally understood one (or more) of the email address listed in ProxyAddresses (EmailAddresses) is assigned somewhere else. Wrote a quick script to find which email address is assigned in duplicate. Here it is.

$conflictingEmail = “user@company.com”
(Get-Mailbox -identity $conflicingEmail).EmailAddresses | `
Where-Object {$PSItem -like “*smtp*”} | `
ForEach-Object {
$e = (($PSItem).tostring()).Replace(“smtp:”,””).Replace(“SMTP:”,””)
$f = “EmailAddresses -like ‘*” + $e + “*'”
Get-EORecipient -Filter $f | Select-Object @{N=”Email”;E={$e}},DisplayName,PrimarySMTPAddress,RecipientType
}