Creation of shared mailbox failed saying "email address already being used" but this is the only Exchange server for the domain

Question

Creation of shared mailbox failed saying "email address already being used" but this is the only Exchange server for the domain

Wildcat63 21

I recently moved a domain from SimpleLogin to Exchange, and I want to create a shared mailbox for ******@moveddomain.com, but I get an error message saying the email address is already in use so Exchange can't create a shared mailbox.

It is true that in SimpleLogin there was an email ******@moveddomain.com, but I deleted that about a week ago just in case that was messing things up.
I tried doing the "Get-Recipient" powershell command and it came up empty.

So I'm stuck. Anyone with any ideas on what needs to be cleaned up?

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Answer accepted by question author

LilyLi2-MSFT 1,986

Hi @Wildcat63

Based on your description, do you mean that the same alias is used in different domains?
If so, the error message is expected behavior, because the alias must be unique within the organization.

We recommend that you use different aliases and display names to create shared mailboxes.

Additionally, if you must want to use the same alias in different domains, you can refer to the document. create-shared-mailbox-same-alias (Note: The environment is Microsoft 365)
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.

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Ash9 96 Reputation points

2026-09-18T03:15:35.1+00:00

Thanks for the clarification. What still feels odd to me is how Microsoft 365 treats address uniqueness differently depending on whether the address is assigned to a user or to a shared mailbox.

A single user can have the same username portion across multiple domains — for example, emailuser1@domain1 and emailuser1@domain2 — without any issue. But if I try to assign emailuser1@domain3 to a shared mailbox, it fails because that username is already used by the user. I’m assuming the user can reuse the username across domains precisely because it’s still one identity, whereas the shared mailbox counts as a separate object.

What’s strange is that email uniqueness is normally defined by the full address (local part + domain), yet M365 seems to enforce a different rule set for user‑assigned addresses versus shared‑mailbox‑assigned addresses. If there’s a rationale or design note explaining why those constraints differ, I’d love to read it — a docs reference would be great.

Either way, thanks for confirming this behavior is intentional, even if the reasoning isn’t immediately obvious.

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Your answer

Ash9 96 Reputation points

2026-09-18T03:15:35.1+00:00

Thanks for the clarification. What still feels odd to me is how Microsoft 365 treats address uniqueness differently depending on whether the address is assigned to a user or to a shared mailbox.

A single user can have the same username portion across multiple domains — for example, emailuser1@domain1 and emailuser1@domain2 — without any issue. But if I try to assign emailuser1@domain3 to a shared mailbox, it fails because that username is already used by the user. I’m assuming the user can reuse the username across domains precisely because it’s still one identity, whereas the shared mailbox counts as a separate object.

What’s strange is that email uniqueness is normally defined by the full address (local part + domain), yet M365 seems to enforce a different rule set for user‑assigned addresses versus shared‑mailbox‑assigned addresses. If there’s a rationale or design note explaining why those constraints differ, I’d love to read it — a docs reference would be great.

Either way, thanks for confirming this behavior is intentional, even if the reasoning isn’t immediately obvious.

Answer 1

Wildcat63 21

I was unaware of this but if you want ******@domain1.com and ******@domain2.com, you will need a unique alias for each in order to create both shared mailboxes. Otherwise you will get the error mentioned in the title to the question.

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