I have 8 or so email accounts set up in Thunderbird and generally speaking I don't get this problem. So that leaves me thinking that Thunderbird doesn't have any intrinsic design or implementation faults (at least with regard to simple sending of messages.)

However I do have amongst my accounts both a yahoo account and a bt account and both are noticeably less reliable than the rest.

BT's email is outsourced, in the main part, to yahoo.com. They started a swap-over to another provider, criticalpath, some years ago, but don't seem to have made a clean break from yahoo. My bt account still links back to a yahoo email account.

In my book, your problem is with yahoo. And between bt and yahoo you have a less than ideal situation where both can take the line that any problem is the other party's fault. I'd recommend you get another email address, independent of your ISP, which you can keep forever and not lose if some day you part company with bt.com.

You should be able to set the bt/yahoo account to forward to a new one (at the server, via the web page) or set your new account to collect from bt on your behalf - but I think this second route might not perform much better than what you already have with Thunderbird.

Oh, one further thought. Do you use anti-virus software? is it scanning your outbound messages? Try turning that off; it may be slowing down the transmission so as to trip a time-out at the server.

Many thanks Zenos - that's exactly what the problem was - Trend Micro's latest update was the problem.