what is the difference?

If it is an alias type thing on Outlook.com or Gmail then the information is replaced with the parent account information when it passes through their servers?

Its the passing through their servers that was the initial problem. The 1st SMTP was through the cable company's email address, which started being rejected by some 'spam' filters just because it used a 'residential IP address range'. So I switched to using our website hosting email address's, but...

Also check that the 'master' SMTP setting in Thunderbird (normally with the server settings of the 'default' ACCOUNT and ensure it is not still usign the Original smtp server

No. It's not. And there is only 1 server on the list to even select from. The good one. I'm thinking I may just have to backup up my emails, and import them back into a clean install. It will take a bit, but not as long as it has taken trying to figure this out.

On the toolbar > Help > CLick on the > then troubleshooting information >

Copy and paste to a message here.

Exactly what problem are you trying to fix here. It sounds to me like you are sending mail from John@myDoamain.com via Google and it is becoming Jean@Gmail.com. If that is the case that is how Gmail works.

My might, and I really mean might get it to allow you. If you go into the gmail setting on Gmail and add the email address you are trying to send from as a permitted address.

In Google settings under accounts and import there is an option for "Send Mail As" if you add the relevant address there you might have some joy.

Thank for all the help everyone. I ended up just starting over. Everything is working as it should now.