I received an email on 11/27/15 and I am still only receiving that same email 4 days later. I use godaddy and I have been receiving other emails after that date on their email server, so when it gets forward to Thunderbird that single email is stuck on sending to me. I have tried to filter it, delete it... As I wrote that, just went to godaddy and deleted the email from there. It stopped sending it. Fixed the problem. Thank you!