And what happened when you contacted BT ?

This is not something you can fix in Thundrbird.

If someone had been abusing your bt email address and sending out a load of surious spam maybe your email address got blacklisted, but it sounds like the BT smtp server is stopping the sending.

Or perhaps it is your IP address that is blacklisted?

TRy these links: https://www.spamhaus.org/lookup/

https://whatismyipaddress.com/blacklist-removal

https://mxtoolbox.com/SuperTool.aspx?

https://community.bt.com/t5/Archive-Staging/remove-blacklisted-IP/td-p/1693376

I use Thunderbird all the time with all my email addresses filed within the program.

When I received a delivery failure from Thunderbird I spoke to BTinternet.com. They asked me to use the webmail or the BT website instead of Thunderbird to email. This should have proved the existence of a blacklist. When using the Btinternet website I could email without rejection. Proving I am not on any email blacklist outside Thunderbird.

When using Thunderbird my Btinternet email get returned but my Hotmail and Gmail email addresses all work properly. This proves the fault is with Thunderbird only.

I redownloaded the Thunderbird program in case the program had been corrupted but the fault or blacklist is still focused on my @btinternet.com email address.

—————- Hello There we have read your message. We try to help you

Modern web browsers such as Mozilla Firefox, Google Chrome, Apple Safari and Internet Explorer use blacklists of reported unsafe websites to help protect users from phishing and malware. When one then surfs to such a reported website, the site is blocked and a warning is displayed. Examples of services that offer such blacklists are the Google Safe Browsing service

Greetings Firefox volunteer

День сумо, you reply is unrelated as it is about safe browsing in web browsers. The Thunderbird email client does not use the Safe Browsing http://kb.mozillazine.org/Safe_browsing .. https://wiki.mozilla.org/Security/Safe_Browsing

The OP thinks his/her email address seems to be perhaps blacklisted while sending emails in Thunderbird but not when going to webmail site in web browsers like Firefox and sending emails that way.

re :They asked me to use the webmail or the BT website instead of Thunderbird to email. This should have proved the existence of a blacklist.

No it wouldn't because it would not be going via the smtp server settings. The email sent from webmail would get rejected by the server receiving the email and it isn't. It is being rejected by BT own spam check - it does not get through the BT server because their own spam check software is using an industry blacklist and it does not like your IP address.

The mail server responded: Sorry, we (that is BT) couldn't send your message. The network address your device has been assigned (that's the IP address ) is on an industry blacklist to prevent Spam.

Have you checked to see if your IP address is on a blacklist using the links I provided eg: spamhaus ?

Thank you for your help everyone.

Problem solved and when explained by yourselves I could understand the whys and wherefores.

I applaud the experts.