Within the Microsoft 365 Exchange environment, when a user creates a booking calendar, the system automatically creates an unlicensed resource mailbox with an automatically generated username (normally user’s firstname+lastname). Emails sent to this resource mailbox are automatically forwarded to the user’s main mailbox.

We recently had an issue with a customer in Oxford where such bookings resource mailbox was forwarding emails to a completely unrelated user (without saving to the booking’s mailbox). Not sure why this was happening, probably had something to do with how the bookings calendar was created originally.

I couldn’t find a way to access the bookings mailbox in the Exchange admin web portal so had to resort to PowerShell.

Booking’s mailbox forwarding settings can checked like for any other mailbox:

Get-Mailbox | select UserPrincipalName,ForwardingSmtpAddress,DeliverToMailboxAndForward

This indeed showed that the emails were being forwarded to the unrelated user in question.

To fix this, all I had to to was to run the following command:

Set-Mailbox -Identity “” -DeliverToMailboxAndForward $false -ForwardingSMTPAddress “”

August 2024
Oxford, Oxfordshire