It is possible that you recently updated Thunderbird. Maybe your firewall/anti0virus product still thinks it should be using the ol version of Thunderbird. So check this out first. Example: I use Norton Anti-Virus/Firewall. Sometimes, after an update, Norton still thinks it can allow only the previous version. I then have to close Thunderbird and access Norton. Settings > Network > Smart Firewall > Program Rules Locate Thunderbird and set to 'Allow' On one occassion, I have had to choose block, then ok and then reset to Allow. If you use another Firewall/Anti-virus program, then there will be similar settings.