I can find lots of information regarding giving users access to shared mailbox calendars, but I seem to be struggling to find information in regards to the opposite. Giving a shared mailbox permission to edit a users calendar (using powershell.)
A bit of background – we moved from on premise to 365 recently, prior to which we had a service account mailbox that was used to automatically update users calendars when they booked annual leave.
After moving to 365 we converted this mailbox to a shared mailbox so as not to consume a user licence. It continues to function correctly for any user that already had the account with editor permissions on their calendar.
Trying to add the permissions to new users with Add-MailboxFolderPermission fails with the error "either not valid SMTP address, or there is no matching information." (however this works for adding normal users)
The odd thing is…. that we CAN add these editor permissions for the shared mailbox manually by using the Outlook client calendar properties…
We would very much prefer it to be automated as part of the new user process.
Best Answer
I tried your steps and got same error message in PowerShell as you shared. The different thing is, I also got error when grant this shared mailbox permission to other mailboxes' Calendar via Outlook client. The error message is "Non-local users cannot be given rights on this server". I tried local mailbox and O365 mailbox. In both scenarios, this shared mailbox (and other local mailboxes) appears as not available:
Another interesting thing is, when i switch the delegate mailbox to the normal mailbox, it no longer say the mailbox is invalid, it works as expected:
So it seems that in Hybrid deployment or Exchange Online, the shared mailbox itself is not supposed to be a delegate to other mailboxes. I read this TechNet introduction to Exchange Online shared mailbox.
With current information, you may want to convert the shared mailbox to a regular mailbox.