It's almost certainly your ISP. Are you a TalkTalk customer by any chance? An almost identical query was posted on their forum yesterday: https://community.talktalk.co.uk/t5/My-Email/Too-Many-Spammy-Recipients-TT406/m-p/1871687#U1871687 What has probably happened is that the server has blocked your effort to send mail to multiple recipients because 'it thinks' you are trying to send spam mail. Incidentally there's another post on the TalkTalk forum raising the same issue, to which a TalkTalk agent has responded saying that the maximum you can send is to 50 recipients per email and 100 recipients per hour. If you've exceeded those limits, there's your answer. If you're not a TalkTalk customer, it's probably the same kind of issue, you'll need to contact your ISP to find out the limits and then keep within them.

thanks for your help and yes i am a talk talk customer. I contacted them yesterday and they informed me the problem was with the third party email provider. As usual from my experience it is talk talk who are the problem. I will try and resolve it with them later today. I am not trying to send anything like the volume of mail stated on their limitations indeed i can't even send a single email to one recipient.

Here is the hint. When it says The Server Responded it is coming from your provider. There are no Thunderbird servers.