Avast 10.3 blocks Thunderbird email sending

Error: 
Thunderbird is unable to send e-mails, because Avast antivirus blocks sending process.

 "Sending of Message Failed, Unable to open the temporary file /tmp/nsmail.tmp, Check your 'Temporary Directory' Setting."


Solution:
Go to Settings => Active Protection => File System Shield Customize link => Exclusions and add *nsemail.html

Source: Avast forum