This isn't a Thunderbird problem. The error message is either from your email provider, or the recipients email provider's server. More likely the latter. Check with the recipient of the message. I'd assume messages to other recipients are not affected?

No. I cannot send to any email address - they all produce the same error message! Occasionally, I can send one or two, then the error reappears. This is clearly a setting somewhere within T'bird which has got amended to a low nimber (1) but also then bars all sending, not just to one address. OR, maybe I have picked up a virus somewhere although nothing shows on checking. Alan

No. I cannot send to any email address - they all produce the same error message!

Then you should talk to your email provider.

Sorry, but they say it is NOT them!

What has happened is that someone on hotmail is using my address as a sender. I cannot know how many he is sending, but I had 29 failed delivery emails returned by mailer-daemon today. This is causing T'Birds volume limiter to cut in, I guess. It is odd that Android/ Blue MAIL does not worry about this while win 10 / T'bird does.

There appears to be nothing I can do other than junk the mailer-daemon returns. So change browser? Alan

What has happened is that someone on hotmail is using my address as a sender.

That's what happening all the time as the sender email can easily be spoofed. It's typically used in spam campaigns. I#m not sure what you mean with 'someone on hotmail' though.

I had 29 failed delivery emails returned by mailer-daemon today.

This sounds like backscatter. https://en.wikipedia.org/wiki/Backscatter_(email)

This is causing T'Birds volume limiter to cut in, I guess.

Thunderbird has no volume limiter. The error message you posted is from an email provider's server, possibly from your email provider.

It is odd that Android/ Blue MAIL does not worry about this while win 10 / T'bird does.

Are you using the exact same server in Bluemail as in Thunderbird?

Thank you for following up. In fact, the problem has now gone away. I changed passwords on my email providers a/c. Did you see the original error message - I am sure that refered to T'bird as the reason for not sending "Too many emails", but I suppose it could have come from Talk Talk, my broadband provider - as I said before my email provider says it is not them!

Still it has gone away and I am grateful to you for helping and sending that reference link. Alan