Sounds like an problem with your anti-virus product. But I do not know what AV you use. First make sure your Firewall (which might be part of your AV product) is not blocking Thunderbird. Perhaps an update occurred in Thunderbird and the Firewall thinks it is a new different program and is currently blocking it. Access the Firewall and make sure thunderbird is allowed. Perhaps your AV software needs to be updated. Do not simply disable AV software as this usually results in the in default taking over. To test if AV is causing an issue: start your computer in 'Safe Mode with Networking', then start up Thunderbird and see if an email will send/download. If you can send /receive whilst computer is using 'Safe Mode with Networking', then start computer as normal and see if: * the AV product needs updating * The Firewall has Thunderbird as allowed. Otherwise: Set up exceptions in the AV software to not scan any Thunderbird folder on startup and not to scan incoming and outgoing emails.