Thunderbird supports one active smtp per incoming account. Under each account's Settings, you can see which smtp server account is associated with it, and change it as necessary.

Each account you add via the new account wizard, Thunderbird should either seek a new custom smtp set-up, or offer you an existing one. For services such as yahoo, gmail, hotmail/live mail/outlook.com, it's generally necessary to use the one they provide. Email services provided by your ISP or some organization such as webhosts may depend on your ISP or email provider offering a "general purpose" smtp server.

To add an smtp "account", go to Tools|Account Settings and at the end of the list, "Outgoing Server (SMTP)" allows you to add new ones.

A common cause of distress is experienced by users who switch between a direct internet connection and a VPN, and need a different SMTP for each environment, and this is an area that doesn't work smoothly in Thunderbird. But I don't think this is your problem.

I do think yopu may have a separate issue with incoming email. If you can see it in the webmail portal but not in Thunderbird, then it would seems that the final delivery from server to Thunderbird has failed. By this point, I'd think it has gone well past SMTP.

Thank you Zenos. I had a good think about being able to get emails and send them from webmail. Therefore it was a TB settings problem, not just SMTP it was incoming settings also. Something had changed them.

         I found the solution. I realised that having added a new email address or perhaps the latest update caused the difficulty. 

• What steps I took; after juggling about with in & out settings changing default. changing Ports and back. I tried to send emails again and again. after closing and reopening TB. TB looked for my password to one email after the other , when I put in my passwords.. hey presto all was back to normal.

So therefore my conclusion is that something hidden had altered or forced an ignore of my passwords. even though in the security window all appeared normal, it was not. (hidden change)

       My changing settings forced TB to ask for password again, and a positive result...

For those who have similar problems , just go straight to security tab/ passwords_ reveal. then copy them in word, not to loose them. Then delete all passwords, close TB and restart then it should request passwords and option to remember them also. I recommend to try this before we pull our hair out ;-) If that fails then copy all in and out settings to word and manually install settings again , restart TB and everything should go well. ( P.S. verify from your ISP what the correct settings and ports are , good luck!!