Opening delegated Mailboxes on Office 365 Hosted Exchange

A few months ago we looked different exchange solutions, in the end our choice was hosted Exchange on Office 365.

Our setup is a little unusual as over time it’s been migrated from SBS through to a full Exchange 2010 product, some of the limitations of SBS meant you couldn’t do generic shared mailboxes, so the normal process was to create a user with a mailbox and then delegate out the required permissions this worked so we never revisited this solution.

The problem came when we migrated the first user with access to delegated mailbox. The user’s Outlook profile would update perfectly, they could access their own E-Mails however as soon as they tried to enter a delegated mailbox they would be prompted for their username and password over and over until their Outlook client crashed.  After some testing we found the delegated mailboxes were the problem.

The question was now whether the delegated mailboxes would work at all, different sources said different things.  However, here’s what I found.

We’ll use two names – Boss and PA_User to help with the explanation. The Boss has delegated out permission to PA_User

Example 1

Boss – On Prem
PA_User – On Prem

As expected the PA_User can see/edit/send on behalf the Boss mailbox.

Example 2

Boss – On Prem
PA_User – O365

The PA_User cannot see/edit the Boss mailbox and will get the Login prompt repeatedly.

Example 3

Boss – O365
PA_User – On Prem

The PA_User can see/edit/send on behalf the Boss mailbox.

Example 4

Boss – O365
PA_User – O365

As expected the PA_User can see/edit/send on behalf the Boss mailbox.

So if you’ve got mailboxes that are delegated out to people you’ll need to move these out to O365 first before you move the users.

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