Hey folks! People on my office network connect to their email via office365 exchange. Several of these users are configured with delegates/send as. One particular user is unable to send as a delegate and receives the error message:

HTTP error. The server cannot fufill the request.

Mail could not be sent. The message has been moved to your Drafts folder.

Account name: "Redacted"

Error code: -18500"

What I have done so far is rebuild the account on the users Mac Mini, reinstall Office all together, as well as check various permission settings over several hours on the phone with Microsoft Support. Sending as the same delegate works for everyone else on the network. Additionally if I configure this particular users email account onto another machine it works just fine. Has anyone else encountered this issue, or have any insight on where i should look next?

Thank you!