Exchange 2016 – Users mail keep being stuck in a Shared Mailbox‘s outbox until someone else uses the Same Shared Mailbox

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I have come across a very unique problem that I have yet to find a solution for as even google failed me.

When User1 tries to send a mail as the shared Mailbox SharedMailbox@ourdomain.com the mail is being placed in the outbox and does not show up in any logs. However if User2 sends a mail as the shared Mailbox, User1‘s mail is also being send out of the outbox with no problem. At this point User1‘s mail also shows up in the logs.

What I have tried:

  • restart Exchange
  • create new mail for User1
  • try different outlook clients on multiple PCs
  • move User1‘s mailbox to a different database
  • assign User1‘s rights freshly (all right including send as)
  • copy the AD user and try it with the copy, which has the same problem as User1)

Seeing as User1Copy has the same problem as User1 I snooped through the attributes in AD and changed every attribute that was different from User2, but still it does not work.

Two days ago it randomly sent out the mail from the outbox without someone else sending a new mail (I checked the logs).

Does anyone have an idea what else I can try (besides making a new AD user for User1) or if I can access the outbox via the Exchange Server Shell?

Best Answer

Check if there is a registry key DelegateSentItemsStyle added on user's client: Email remains in the Outbox when you use the DelegateSentItemsStyle registry value

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