An error occurred while sending mail. The mail server responded: relay not permitted.

Are you using the email service of your ISP?

The email service. Vic

The email service.

Assuming this was a 'yes'.
Make sure to be connected to the Internet using your ISP when sending email using your ISP email account. If you're connected to the Internet via a different ISP (e.g. a public WiFi), you most likely get that error.

Also make sure not to use a different email account with a different email address, and trying to send a message via your ISP's email server.

Yes to these questions. It has only recently become a problem and is intermittent. Vic

I'd suggest you talk to your ISP/Email provider.

Thank you, but I'm talking about the email provider - Thunderbird. I don't think it's a problem with my ISP, but I can try them.

Vic

I'm talking about the email provider - Thunderbird.

Thunderbird is an email client. Neither Thunderbird nor Mozilla are an email provider. Well known email providers are e.g. Gmail, or Yahoo.
Your email provider is where you get your email address from. In your case your email provider is identical with your ISP.
At least that's what I assumed from your 'The email service.' response above.

OK thanks for your advice. Vic

I spoke to my ISP this morning. He took me through a few checks including checking the details of the SMTP. He suggested Encrypting Password in Account Settings/Outgoing Server but that didn't help so went back to Normal Password. Again I was able to send some emails without fault, however it has since been a problem, again. The ISP tech support then commented that if it happened again, it must be a problem with Mozilla Thunderbird and I should contact them.

It seems odd to me that I cannot contact Thunderbird - I seem only able to contact helpful "Support Forum" people like yourself.

Not sure what to do next, other than change my email to Gmail or Outlook. Best wishes Vic

He suggested Encrypting Password in Account Settings/Outgoing Server but that didn't help so went back to Normal Password.

Whoever you talked to had no clue. If they suggested to try the 'Encrypted Password' setting, the server needs to support it. They should know whether there own server does actually support it.

The ISP tech support then commented that if it happened again, it must be a problem with Mozilla Thunderbird and I should contact them.

As said before, a completely clueless support person.
The error message is from the server, not Thunderbird. Since this is intermittent, you didn't change anything in your server settings, and you're connected to the Internet via your ISP, logic suggests the problem must be on the server side.

I suppose your best bet is to escalate the problem to the next support level at your ISP/email provider.
They should be able to check the logs of their own server to get an idea what goes wrong. I'd admit getting through to the person capable of check the server logs will be a challenge.