What is the exact error message?

thanks for your reply - there is no error message. The progress bar advances to 100% then sticks there. If I close it - "sending message failed" comes up. If I leave it, I get the message "connection to server timed out, sending message failed". in either case, the recipient does get the message. This only happens to messages with attachments however small.

thanks for your reply - there is no error message. The progress bar advances to 100% then sticks there. If I close it - "sending message failed" comes up. If I leave it, I get the message "connection to server timed out, sending message failed". in either case, the recipient does get the message. This only happens to messages with attachments however small.

Try to start *Windows* in safe mode with networking enabled. - Win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01 - Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode - Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

thank you - in safe mode the message is sent normally but when the computer is started normally it is still there. I should add the problem only started a week ago. Before that I had been using Thunderbird for years with no problem.

That points to anti-virus software as the culprit. In your anti-virus software create an exception for the Thunderbird profile folder, so that it will not be scanned by your anti-virus real-time scanner. For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also, don't let your anti-virus software scan incoming and outgoing messages. And don't let it scan attachments.

thanks a lot. now I cat know my messages have been sent. I landed 5 identical messages with large enclosures in an unfortunate friend's inbox last week.