Do you use Anti-Virus product 'Norton 360' ? If yes: It is possible that Norton got confused by the Thunderbird update and thinks it is a different program, so a port is being blocked. Try this first. * Exit Thunderbird - this is necessary. * Access Norton * Under 'Settings' > 'Firewall' > 'Program Control' * scroll down until you locate : .............Applications\Mozilla Thunderbird\thunderbird.exe It should say 'Allow', but select 'Custom' from drop down option. A new window opens: Does it say exactly the following: * Thunderbird, Allow, Direction: In/Out, Computer: Any, Communication: Any Protocol: All And is the checkbox selected with a blue X ? If no, something is different from the above, eg: Any Protocol does not say 'All' * click on 'Modify' * Actions tab - select 'Allow * Connections tab - select last one -'Connections to and from other computers' * Computers tab - Select 'Any computer' * Communications tab - select 'All' * Advanced tab - select If explicity requested * Description tab - Type Thunderbird * click on all the OK's * Then make sure it says 'Allow' If it says all of that already: See if the following forces a resync - select 'Block' and click on Apply and then cick on 'Close' and exit Norton. Then access Norton Firewall again, look for Thunderbird as before and select 'Allow', 'Apply' and 'Close' Now start Thunderbird. There have been reports from people that forcing a resynch sorted out the problem