What OS are you using ? What version of Thunderbird are you using? What Anti-Virus are you using? Is Thunderbird in online mode or offline mode ? Look in bottom left corner of Status Bar - if icon has a red line through the icon you are in offline mode. Clickon that icon to revert to online mode. Is Thunderbird set up as an allowed program in the Firewall settings ? Exit Thunderbird. Access Firewall which might be part of Anti-Virus product. Thunderbird must be set as an allowed program. If it is already set as allowed, then block it, ok it. Then access Firewall again and reset as allowed and ok it. Sometimes a Thunderbird update gets seen as a different program by the Firewall, so it ends up getting blocked. The resetting can fix. What server ar you talking about ? gmail , yahoo, AT&T, etc ?