Is it any reipient? a special mailserver? your mailserver?

Check your server-setup

Thanks for your prompt reply - much appreciated. Regarding recipients - it's not just a specific recipient but a variety of people I have communicated with in the past with no problem.

Regarding mail server, I'm not sure what this means. I use Thunderbird to send/receive several different email accounts (e.g Googlemail and from my own domain). As far as I can recollect, it is just my own domain account that has the problem. Snapshot of the server settings enclosed, if that helps clarify the problem.

Cheers.

You are having problems sending but you post your inbound server setting. That is not much help. From the Help menu select Troubleshooting Information Clear the checkbox to show account name so no personal info is displayed. Click the button to Copy to Clipboard. Come back here and paste that into a reply window.

OK - is this what you mean? (There is a lot of data in the report)

 Application Basics

   Name: Thunderbird
   Version: 31.4.0
   User Agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20150109111741
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

This setup guide indicates that you need Normal Password set. You seem to have all the accounts set to None http://community.plus.net/library/email/email-setup-mozilla-thunderbird/