re :Restart with add-ons disabled with no luck. Tried plain text and html emails but same results.

This is not something that is being created in Thunderbird. It does not sound like you personally inserted the link via Thunderbird Write window or used a pre-created template email. It is not a Thunderbird 'addon' extension.

It is something you enable via the SMTP2GO control panel. https://support.smtp2go.com/hc/en-gb/articles/223087607-Unsubscribe-Footer They are adding the link to emails as they pass through their server; they detect it is you and add the link.

Our unsubscribe functionality allows you to easily insert an 'unsubscribe footer' at the bottom of all of your emails that have been sent from a particular user.

https://support.smtp2go.com/hc/en-gb/articles/115004408567-Sender-Domains

As you are using an imap server, you will see emails as they appear in webmail account which is after the server has applied various settings.

Does the new Write message which you initially compose the email actually show the inserted link before you click on 'Send'?

The unsub footer link is put on by SMTP2GO as it passes through their server so I only see it when it is received by my personal accounts. The previous owner of the account who uses Thunderbird has had same issues for a while too as I saw from archived messages. I put Thunderbird on a second machine and same results. SMTP2GO tech support went as far as put Thunderbird on their machine and had the same results and send screen captures. According to SMTP2GO they are not seeing this issue with any other email programs. I will try another email program but hoped Thunderbird would have worked. I see it has auto updated since installing recently and believe it caused the issue. Thanks.

walleyeanglers said

The unsub footer link is put on by SMTP2GO as it passes through their server so I only see it when it is received by my personal accounts. The previous owner of the account who uses Thunderbird has had same issues for a while too as I saw from archived messages. I put Thunderbird on a second machine and same results. SMTP2GO tech support went as far as put Thunderbird on their machine and had the same results and send screen captures. According to SMTP2GO they are not seeing this issue with any other email programs. I will try another email program but hoped Thunderbird would have worked. I see it has auto updated since installing recently and believe it caused the issue. Thanks.

Lets look at this in a sensibly way.

Thunderbirdc ends it's involvement in the generation and handling of the email at the time it hands it off to the SMTP server.

Then the SMTP server modifies the body of the email to insert some code that is obviously causing all sort of issue.

Perhaps someone for the provider could offer some insight into why they code they add after Thunderbird hands it off is causing an issue. Personally I digitally sign my emails to prevent that sort of email tampering, it is not benign. What I send should be exactly what the recipient gets, not some sort of facsimile.

So the problem is SMTP2GO. They are intercepting your perfectly ok outgoing mail and adding "footers" to your emails because you have selected that option ans it is assumed you have done it all correctly and they are making a mess of the result which is not doing you any favours and they claim it is a Thunderbird issue and want you to jump through a few hoops with trying out various email clients because they are not up to job they claim they can do.

Sorry, but I'd be more concerned about SMTP2GO. After all, you are paying for a service they do not seem to be able to provide and blaiming someone else for their shortcomings is not my idea of a good service.

I certainly would not be happy about anyone tampering with my emails because at the end of the day, it's my name that I'm risking, not an smtp server offering various tracking services.

Found TB to have problems reading ANY link in an email without SMTP2GO interaction. After installing TB on a second machine and have it work properly, removed (including profiles) and reinstalled on main machine to have it work successfully. Now to restore emails.