Thunderbird's latest version has a problem. Turn off Email attachment scanning in SecurityCenter > Virus and Spyware Protection > Real Time Scanning > Settings
You are still protected if you save said attachments to the hard drive, they will then be scanned.
This thread has some insight on that - https://community.mcafee.com/thread/50903?start=0&tstart=0
Unfortunately, disabling the scanning of email attachments is not an acceptable solution. This use to work until one of the most recent McAfee updates.
This problem occurs even when there is NO attachment.
I would expect McAfee to fix this problem rather quickly. I view disabling the scanning of email attachments as a short-term work around. "Short-term" is to be taken as a fix is ready within the next 10 days.
If you want it fixed quickly make sure you have added your voice to the cases reported by contacting Tehcnical Support.