I do not know Plusnet, but I suggest using full email address in SMTP server, not just email prefix.

Thank you but I'm not sure what you mean - the smtp setting has always been 'smtp://relay.free-online.net', how do you suggest I add my email to this (my email address being xxxxxx@xxxxxxx.free-online.co.uk)? (sorry not sure what you meant by email prefix).

The error message you posted gave me the impression that your entry for 'user name' for SMTP server is gfinley, not the full email address. If true, most email providers require the full email address there.

Oh gosh I hadn't realised that screenshot showed my details. I've now deleted it, and please delete my user name from your post, I'd appreciate it. I've tried your suggestion but it didn't work, I got the same notif. as before (ie.it failed with my email address as a user name), plus the one I'm attaching now (Send message error). I entered the same password as it shows on the Saved logins settings, and the Authentication method is Normal, as it used to be (and as it is with my other email addresses).

Well, there has been some improvement; you are not getting the 'unknown sender' error. That is why it is important to share exactly what thunderbird is seeing. Screenshots do that. Otherwise, people here have no idea of the problem. It was in your posted error that I was able to deduce part of the problem. You are receiving an error that there is an error in the SMTP configuration, yet you refuse to share exactly what it is. I see one of your attempts to fix this was changing password. All I can suggest is to experiment with different SMTP settings, such as SSL/TLS and port 465, STARTTLS and port 587, normal password. If no combination works, you may have a password problem. You can select settings>privacy&security, click savedpasswords and then showpasswords. That lets you see whatever password thunderbird is attempting to use.

Thank you for your reply. Re: "You are receiving an error that there is an error in the SMTP configuration, yet you refuse to share exactly what it is." >>> We are always told not to share personal info in social media, forums, etc, which is the reason why I was being cagey (please could you delete my user name from your post, thanks in advance). >>> I have re-instated my usual User name in the SMTP Server settings (as my email address didn't work, see my last post). >>> I am attaching a screenshot with the User name, ie xxxxxxx left blank >>> I tried to send an email but again got the 'You are receiving an error that there is an error in the SMTP configuration', see attached. >>> I have checked my Account settings, as follows: - Account name: my email address, xxxxxx@xxxxxxx.free-online.co.uk - Your name: Xxxxxx X Xxxxxx - Email address: xxxxxx@xxxxxxx.free-online.co.uk - Outgoing server (SMTP): Free-online - relay.free-online.net - SMTP Server: as 1st screenshot below (with Username xxxxxxx)

>>> I changed the SMTP server setting to SSL/TLS and port 465: gets stuck on 'Sending message'. >>> I went back to STARTTLS and port 587: get Send message error as before (2nd attachment). >>> Both the above with Normal password >>> I have checked my passwords in the Saved passwords, and my saved password is correct (I assume it is as Thunderbird fills my Inbox).