I’ve had a confounding problem — we have sendmail magicing up millions of e-mail addresses for us, but occasionally we need to be able to send from one of these addresses too. I’ve got a web form that allows text-based messages (html or plain text), but I don’t want to figure out how to upload and attach images via a web form. Until I get around to updating the web form, I just set up a new Exchange mailbox and grant myself full access (which includes send as permission)

Except ever since we got always updating Office 2016, I’ve gotten nondelivery reports when I subsequently try to send from this new mailbox that claim I don’t have permission to send as the user in question. And I’ve verified my access three times. Even added explicit send-as in addition to full mailbox access.

I’ve finally discovered why I get this false error. The ‘from’ in Outlook allows free-form text which then may or may not resolve against an Exchange mailbox. And based on the permissions of the mailbox (or the lack of permissions of the non-resolved mailbox), it may or may not work. So I don’t have permission to send from newmailbox@ourdomain.ccTLD, I do have permission to send from the Exchange mailbox that happens to have that as its primary SMTP address. Sigh!.

When you use offline mode / cached Exchange mode, and an offline address list, the SMTP address doesn’t resolve out to that mailbox. And Exchange quite properly reports an error. To get the whole thing to work (assuming “wait until tomorrow” isn’t a good answer):

First, the offline address needs to be updated (either wait or hit the powershell management console on the server)

Update-OfflineAddressBook -Identity "Default Offline Address Book"

Secondly, Outlook needs to retrieve the updated address book. Within the Outlook client, use send/receive to update the address lists. Then you can send as the mailbox to which you have perfectly configured access.