IMAP4 IMAP is an incoming mail protocol, so I guess you do not have the server settings correctly installed. I think it is a very surprising thing that reflected very badly on your provider that they could not identify you were trying to send mail using the receive protocol. Consider the quality of that support when you pay their bills.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Please also add the result of these troubleshooting steps to your post

• Does Thunderbird work in TB Safe mode (see Thunderbird Safe Mode)?
• Do you use anti-virus and firewall software? What is the version?

I am due to see my server provider on Monday and I am sure that they will be very interested in your comments. I have used them for more years than I can remember and, previously, they have sorted all my misguided attempts to put in incorrect IMAP information and as far as "we" can see, everything is as it was previously when it worked OK. My AVG is up to date and they have dropped the firewall and anti-virus to see if that had any effect but to no avail. I will try your suggestions with help and safe mode. Thanks.

IMAP is the server you receive mail from. SMTP is the server you send mail through. Your complaint is you cannot send but you continue to talk about changing settings on the IMAP setup. That is not going to help your problem. You need to fix your SMTP settings.

where is the troubleshooting information?

As airmail said your SMTP settings are clearly wrong. you are connecting to a server that thinks you want to get mail not send it. we might stand a chance of advising you is we had the troubleshooting information, but without it all we can say is it is your SMTP settings are obviously wrong.