It would help if you posted an image of the error message OR posted all the error message - word for word. # What OS are you using ? # What version of Thunderbird are you using? # Are you talking about a Pop or Imap mail account ? # What server are you trying to connect to? # Are you using a 2 step verification / app generated password ? # What Antivirus product are you using? # Do you allow that Anti-Virus Product to scan A) Profile folders B) incoming mail? Post the following info: In Thunderbird * Help > Troubleshooting Information * scroll down to 'Mail and News Accounts' section * Post an image showing all contents of that section OR copy all the text and post in this forum. Try this and report on results. * Right click on Inbox and select 'Properties' * Click on 'Repair Folder' * click on 'OK'