I have come upon a fix, somewhat specific to Eudora, that allows it to send mail while keeping SSL enabled. I am back in business.
Relatedly (and more generally), the discussion at this StackExchange thread opines that Spectrum has upgraded its server SSL level to something called "TLS 1.1+". The meaning of that is above my pay grade, but I suspect (a) Spectrum isn't going to undo the change, and so (b) it will be up to the user to get your client software to play with it.
I've been unable to send mail since last week (incoming worked OK). Eudora reported an error of "Server SSL certificate rejected".
Today, after selecting "never" for "Secure Sockets when sending" (<tools - options - sending mail>, I am able to send.