I'm running Thunderbird 45.4.0 on Windows 7. Thunderbird has worked great (for years!) until this morning. After starting Thunderbird in normal mode, it hangs at "Connected to <mail server>" or "Checking mail server capabilities". In Thunderbird's safe mode it hangs at "Checking mail server capabilities". Mail stored on the computer is accessible, but attempting to access mail on the server (e.g. sent mail) returns "Unable to connect to your IMAP server..." All three of my accounts are stuck this way. All accounts work correctly via the ISP's webmail so the issue is not on the server end. I've rebooted the modem and router and turned off the firewall with no effect. Other network capabilities work OK (e.g. http, https, ssh, ftp). Unsurprisingly, attempting to send an email also fails with "The message could not be sent because the connection to the Outgoing server (SMTP) <server name> was lost in the middle of the transaction. Try again." Any hints will be appreciated.