Perhaps if you were to share the content of the failure notice someone might be able to more that just guess. BTW I would not jump to Thunderbird being the issue, I do not discount it as a possibility, but recent experience in the mail space would indicate Microsoft trained technicians are often responsible to some very odd changes that make no sense Beyond that, even an email sent via Thunderbird using Google actually uses a completely different path to mail sent from their web interface, and the Thunderbird mail has to navigate the hurdles of broken cryptography caused by antivirus product and their habit of amending mail on the fly which can lead to deliverability issues. SO at this point we know it is not working, but we need lots more information to start to determine why that may be.