When attempting to send an e-mail, you get a non-delivery report containing the following error code:
Usually, this error is caused if you are sending mail on behalf of someone else, and don't have permission to do so. Note that send on behalf and send as are two different, exclusive permissions. Having one permission does not automatically allow the other.
In rare cases, this error can also occur when a user's mailbox has been migrated to Exchange 2013, and the shared mailbox is still on the old Exchange server. This causes problems with permissions, and can quickly be resolved by setting the required permissions on the shared mailbox (send as/on behalf), then completely remove and recreate the Outlook profile.