Outlook/Exchange NDR: “Delivery has failed to these recipients or groups” for user with Send-As permissions

Had a user today that was receiving an NDR when trying to send as another user. This was strange as this user has always had full mailbox access and send-as permissions to the mailbox in question, so the NDR was unexpected.

It turned out to be an issue with cached address entries when typing the sender’s address into the “From” field in Outlook. I removed all of the relevant entries and then resolved the sender from the address book, and the email went through without any issues.

One for the memory banks…