Answered: Rebex.Mail.MailMessage - Save as Outlook message to be sent - From address issue

Can you please verify that you are still able to send the mail 'manually' as before:

When I remove the pre-filled email address and manually type in the
exact same address, I am able to send the message, indicating I indeed
do have permission to send on behalf of it.

Can you please verify that you are able to send the mail using old version of Rebex library. It can be downloaded from https://www.rebex.net/protected/DownloadArchive.aspx
Back than you probably used version 2012 R3.

If you are able to send the mail manually. Please,
a. locate the mail in 'Sent Items'
b. save it to a .msg file
c. ZIP the .msg file and the original .msg created by your app
d. send the ZIP file to support@rebex.net for analysis