M365 – Adding new user error – “We’re sorry, it looks like that email address is already in use. Please use a different email address.”

Trying to add a new user to the 365 portal returned the error that the desired email address was already in use, but helpfully didn’t tell you which account/mailbox it was in use with. To identify the existing mailbox that is using the address in question, launch PowerShell then run Connect-ExchangeOnline command, logging in with admin credentials, then run the following command:

get-mailbox | where {$_.EmailAddresses -match “<email address>”} | ft Name, RecipientType, EmailAddresses

[NOTE: Replace <email address> with the address you want to use]

The returned result should allow you to find the existing mailbox utilising the address and then you can decide how to proceed!