Assuming your question is 'How to access the SMTP server settings', the answer is: - click to 'account settings' pane - click the account name in left sidebar - then, click the 'edit SMTP server' button in lower right of screen.

Sorry David, that is not the question at all. What you describe is the method I used to enter the data that I recorded last time, but which failed to solve the problem.

I'm thinking the Help Desk is steering you wrong. Port 25 is not the standard port for a secure (SSL) connection to an SMTP server. The default port is usually 465. Port 25 is usually only used for an unencrypted connection.

I can't find any official care4free.net help page with suggested settings. I think you should first try setting the port to 465 with SSL/TLS or STARTTLS (try both.) If that doesn't work, try port 25 with no security (this is just for a test, not recommended to keep.) If one of those works, let us know.

(I also found an old post suggesting port 587 "with or without security", so if the above doesn't work, try this.)

This is your email service's problem, not Thunderbird's. In the absence of clear published settings recommended by care4free.net or plusnet, I think you're just going to have to juggle the settings around until you find the right combination.

According to Plusnet, try relay.care4free.net, port 587, STARTTLS, normal password, email address.

https://www.plus.net/help/email-guides/how-to-set-up-plusnet-email/

https://www.plus.net/help/email-account-changes/

I just spent several minutes writing a reply, and when I clicked "post" received a notice "Bad Gateway 502". Is there anything you can do to prevent this, please?

Will try again to summarise, and copy to clipboard before clicking "post" ... Using relay.care4free.net (I thought you had the answer because that rang a bell) with port 587 and SSL or STARTTLS I was told I could not connect, connect error 10060. Trying other combinations, I was told after 10 seconds or so that the connection timed out. Also tried again Lin's idea port 465. With some combinations I was told the connection timed out, with others the "sending" display simply hung for a number of minutes, until I cancelled it. As regards whose problem it is, my conversation with Plusnet help desk assistant lasted more than half an hour: he spent most of that time studying guidance notes on your web site, before admitting that he could not solve the problem and suggesting that there should be on my Account Settings" page an option "Account Actions" through which I may access a solution.

I just spent several minutes writing a reply, and when I clicked "post" received a notice "Bad Gateway 502". Is there anything you can do to prevent this, please?

No, it's a server-side error and out of your control. It indicates a temporary web server problem. It happens to all web sites occasionally.

Just a tip: when writing something long and complicate on a web forum such as this, I've learned the hard way to compose the response in a text editor such as Windows Notepad, then when I'm ready, copy and paste it into the forum window. This is to avoid mishaps like what you encountered.

The second link in my reply contains this:

"For... Care4free... we are enabling the ability for you to use the Plusnet server details to send and receive email using SSL/TLS. You must ensure that the username is configured as your full email address."

If that's been enabled, the servers would be imap.plus.net, 993, SSL/TLS, normal password, email address, and relay.plus.net, 587, STARTTLS, normal password, email address. The staff at Plusnet should (but don't be surprised if they can't) be able to verify this.

Thanks again folks for getting in touch (the message that was wiped out by the Bad Gateway began with similar courtesy). sfhowes you are almost complete genius: relay.plus.net rang a louder bell than other ideas, after trying your suggested settings and being told that the server timed out, I tried changing the port number to 465 and at last was asked to enter my password (I had been baffled that no other attempt had caused such a request), and I can at last send mail. One thing still puzzles me. My two reserve (Android) devices, using different email apps, both automatically detected the correct settings, so I don't understand why Thunderbird does not do likewise on Windows. Plusnet are in the process of transferring this part of their business to Greenby, so if I live long enough for there to be a next time, the required settings may have changed: let's hope for the better! Once again thanks for your efforts (I am sure a donation will follow ....)