Unless you want to be a spam magnet it usually isn't a good idea to post your full email address to a public forum.

It will not accept my plusnet username.

Please explain in detail

• what you're doing
• what happens
• what you expect to happen

Even entering all the other data by hand makes no difference.

With 'all the other data' I suppose you mean the server settings for Plusnet. Did you guess what the correct settings may be, and unsuccessfully tried arbitrary combinations of different parameters? Or did you check about the correct settings with Plusnet before?

If you haven't checked with Plusnet before, please do so, and post the correct settings here.

Point taken - foolish to quote an address! Not a savvy thing to do. First, my quote 'all other data' meant server settings copied from the servers web site & also compared them with settings already being used on a windows 10 tablet running Windows Live Mail. Filling in the new account form 1st. Enter a Name. 2nd. enter full email address. 3rd. Enter password. It then goes off to load the other parameters. I then alter those to the true values and click on the test button. It then comes back with the error message. Hope that is accurate enough?

I then alter those to the true values and click on the test button.

Are you saying you deliberately modifying the values the wizard sets up? If so, why?

And what actually are the 'true values' you entered?

Once again, please post the correct settings as per Plusnet instructions here.

After my original clanger, I am struggling to answer without compromising myself as before. So after filling in the 3 boxes on the first questionnaire (name, email address & password) Clicking 'CONTINUE' brings up another screen with the boxes already filled in, which do not agree with the ones given by PlusNet. These are for a POP account:- Incoming server - mail.plus.net Incoming port - 110 Outgoing server - relay.plus.net Outgoing port - 587 (recommended) or 25 SSL/TLS - No SMPT Authentication - No (only needed if you are sending emails from a non PlusNet internet connection.) After altering the incorrect boxes, clicking on the 'TEST' button, the error message which follows, ends the sequence. Trust this a little more coherent.

What is your anti-virus software?

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the account setup work?

Wrt Plusnet, their security is non-existent. Everything is transmitted to the server in the clear, including your password. If I were you I'd run and find a new email provider.

First, May I thank you for the effort in trying to solve my problem. After many attempts using Safe mode (which I could not get with internet connection) and other ways of sneaking data into the system – I failed. I reluctantly accept that Thunderbird & I are not compatible. In closing this query, your redaction of my email address is greatly appreciated. Also your comments about my provider would seem correct, but also, comparing it with a few others, security appears flimsy with all of them. Thank you.

After many attempts using Safe mode (which I could not get with internet connection)

Not sure what exactly was the problem. If you use a WLAN for Internet access, this may not work in safe mode. So you may need a wired connection to your Internet router.

... security appears flimsy with all of them.

Not sure which providers you looked at. In any case, Google may not be the best choice in terms of privacy. However, in terms of security, there is not much left to be desired. The bare minimum today would be TLS encryption for communicating with the server.