What are your settings? They suggest:

incoming: mailc50.megamailservers.eu on 993 (IMAP) or 995 (POP), SSL/TLS, normal password, email address

outgoing: mailc50.megamailservers.eu on 465, SSL/TLS, normal password, email address

If you have a VPN, disable it. Some non-MS antivirus apps can also block connections.

Yes, these are the settings Easily gave me. Not sure what a VPN is - I just have a laptop, two wired external drives, and an air connected printer. No non-MS antivirus.

Post a screenshot of the error you get, or describe the exact wording.

Check that you've followed the instructions for registering your domain and enabling email.

Latest attempts now produce this result. Clicking retry flags up 'Incoming server already exists' Domain registration is fine and has forwarded emails perfectly for years until now, and I can still get them through Easily's Control Panel. I'm not very techy and am probably missing something simple.

It looks like you're trying to add an account that has already been added (view it in Account Settings). If adding the account will fix things (doubtful), you need to remove the existing one (Account Settings/Account Actions), restart TB, add the account. But you still haven't described the failure or error with the existing setup.

I have deleted account and started again - same difficulty. After filling in the settings I get message "Unable to log in at server. Probably wrong configuration username or password" But as I mentioned previously, these details work fine if I bypass Thunderbird andI go directly to Easily.

And I have tried deleting and reinstalling Thunderbird, but still same error message.