What is your anti-virus software?

I use Bullguard on WIndows 8.1

Try to start *Windows* in safe mode with networking enabled.

- 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

- XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still in Windows safe mode, start Thunderbird in safe mode.

- https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

I disabled the antivirus part of Bullguard and the error disappeared. I sent about 7 or 8 test emails and no error message. I hope you have a fix because I am paid up on bullguard on five computers for two more years.

Don't let your anti-virus software scan your Thunderbird profile folder. Don't let it scan incoming and outgoing messages. Don't let it scan attachments. Don't let it intercept your secure connection to the server. Remove any extensions it may have installed for Thunderbird.

That is too extreme. This error is annoying, but it is not worth disabling Bullguard. I went into the advanced options, but I cannot find a way to removing scanning on the smtp. Instead I went into the firewall section and I changed it to incoming only and disabled smtp. I sent 6 emails without the error. I will monitor to see if this fixes it. Thank you for putting me onto my anti-virus. You were a big help.