Sending of message failed - connection timed out.

I have a similar problem to others reporting a connection time-out. But the circumstances now seem different. It is getting worse. First thing today, I noticed that Thunderbird emails (current/up-to-date version) were taking several minutes to finish sending. After sending 3 successfully, but slowly, all further emails failed, as per image attached. My wife's computer, using the same email provider, etc, was working okay. I checked and compared the account setting, and they were slightly different (no idea why) so I altered mine to match the other PC. That didn't make any difference anyway. Then (a few hours later) my wife's PC started having the same problems as mine, as if it's a spreading disease. Could it be a virus? Thanks.