Unable to access SharePoint Online site mailboxes

Recently I ran into an issue with SharePoint Online where I could not access site mailboxes. Effectively any site mailbox I clicked on in SharePoint would give me an error stating:

Guest access isn’t supported
It looks like you are currently signed in as a guest user. Site mailboxes do not support guess access.

“Oh dear, how terrible, something didn’t work for you precious” I hear you say sarcastically…. so let’s look at the circumstances and see what you think afterwards:
– The site mailboxes were within my production site collection
– Everyone else in the organisation could access them

“Whoop de-do, how did you survive?” I hear you continue to say sarcastically. Let’s continue:
– I couldn’t even access site mailboxes that I had created
– I am a site collection administrator for our production site collection
– I am an Office 365 global administrator for our tenant

“Oh…” yes, I thought so.

We investigated this ourselves and couldn’t see anything past the correlation ID I was being given.

Working with Microsoft it took a couple of escalations to get to an engineer who could help out. He worked out the root cause was that my account was flagged as a guest in the user info table as a result of External User accounts (me) existing in other tenants. When the username was resolved to be added/updated in the user info table it was marked as a guest. Ultimately SharePoint Online was resolving to my External User first, not my AD account.

What was the fix? Unfortunately nothing simple. The escalations team had to manually perform a back end change to reset the flag. Once the flag was reset my access was restored.

I’ve been told there is a service change in the works to address this issue going forward, but the release date is not known at this point.

Sorry this wasn’t exciting – I just hope this helps someone else in the future. Kudos to the engineer who identified and fixed it for me, some top quality work and a speedy resolution.