Hello, verify that your mail server supports the minimum version of TLS. Thunderbird 78 expects higher security standards for the so-called TLS network encryption protocol. If your provider does not support the newer versions of the protocol, sending and receiving of messages may fail. You can temporarily lower the bar to check if this is the reason for your problem (which obviously reduces security, so it's not recommended). More details and instructions here: [[Thunderbird 78 FAQ]]