Outlook Autodiscover Caching, MAC OS

hi JMyers404​ This does sound very much like an Autodiscover / mailbox identity caching issue, and you’re not imagining things, I’ve seen similar behavior specifically with New Outlook for Mac.

A few observations and answers based on real-world experience:

What’s likely happening

New Outlook for Mac is much more service-driven than the legacy client. When an SMTP address previously existed as an alias, Outlook appears to cache the mailbox GUID / identity mapping server-side (not just locally). Even after the alias is removed and a brand-new mailbox is created with the same SMTP, the client still thinks that address is already bound to the old mailbox identity.

That would explain why:

• OWA works immediately (fresh server-side resolution)
• New Outlook for Mac insists “This account has already been added”
• Local actions (Keychain cleanup, reinstall, profile reset) have no effect

At that point, you’re fighting a service cache, not a local one.

Is there a supported way to force re-resolution?

Unfortunately, today there is no supported client-side method to force New Outlook for Mac to forget or rebind a mailbox identity when an SMTP address was previously used as an alias.

The usual things people try (and you already did):

• Removing all accounts
• Resetting Outlook
• Clearing Keychain
• Reinstalling Outlook

.don’t help because the conflict is upstream of the client.

Is this a known limitation / bug?

Yes, while it’s not well-documented publicly, this is a known edge case with:

• Alias reuse
• Cross-domain mailbox moves
• New Outlook for Mac (and sometimes mobile clients)

The old Outlook for Mac and Outlook for Windows tend to be more forgiving here. New Outlook is stricter because it relies heavily on immutable mailbox identifiers.

What usually resolves it, In most cases, one of these works:

• Time: wait for backend caches to expire (can be 24–72 hours, sometimes longer)
• Temporary SMTP change: assign a different primary SMTP to the new mailbox, let it fully settle, then switch back
• Microsoft Support ticket: ask for backend cache / identity cleanup (this is often the fastest guaranteed fix)

Practical recommendation, If this is a scenario you expect to repeat:

• Avoid reusing SMTP addresses that previously existed as aliases on other mailboxes
• If reuse is unavoidable, plan for a delay or temporary address during provisioning
• Test with Outlook Web and Windows first before declaring the mailbox “ready” for Mac users

You’re spot on that this isn’t a configuration issue on your side, it’s a New Outlook for Mac + Autodiscover identity edge case. If you open a support case, mentioning mailbox GUID caching after alias reuse usually gets it escalated faster.