Problem
You have a hybrid Exchange environment and you need to grant SendAs permissions for a user on a mailbox.
The location of the mailbox matters here as the commands differ depending on whether the mailbox is in the cloud or on-premises.
Note:
You must have the correct Exchange role permissions to apply mailbox permissions.
Solution
Important. You must have the Exchange role access that allows this.
Exchange Online
Add-RecipientPermission `
-Identity "Fakey Shared Mailbox" `
-Trustee "Fakey McFakerson" `
-AccessRights SendAs
Exchange On-Premises
Add-ADPermission `
-Identity "Fakey Shared Mailbox" `
-User "Fakey McFakerson" `
-ExtendedRights Send-As