Do you have an antivirus product that might be scanning outgoing email and delaying it? Do you have a firewall (hardware) that is blocking the ports used to send mail. Generally software firewalls like those in antivirus products block programs while hardware ones block protocols and physical ports. Increasingly ISP's are offering basically crippled hardware to connect with (which has ports disabled etc) as it reduces the support profile of the hardware.

There is also the issue of VPN's. They are becoming increasingly popular for some reason, but they frequently cripple sending mail, or mail generally. The SMTP server that knows from your account settings you live in Baltimore refuses the send request from China, where the VPN randomly drops you out of their tunnel. (Every connection from a different place is the whole basis of VPN security through obscurity and plays havoc with some of the person protection routines deployed by banks, governments and big internet players to safe guard connection security to their servers.)

I don't have any antivirus products scanning my emails and my device firewall doesn't block thunderbird, nor does my router's block the needed ports. I also don't use a VPN. For some weird reason I was suddenly able to send emails today.