Apple Mail - SMTP & SSL fail

On Friday sometime about 7pm Eastern US, my Apple Mail stopped being able to send mail. This is across several email addresses on multiple providers (including iCloud, Verizon/AOL, and others). The Mail Connection Doctor shows every SMTP account connection failed with:

Could not Connect to this SMTP server. Check your network connection and that you entered the correct information in the Account preferences. Also verify that the server supports SSL. If it does not, deselect the "Use TLS/SSL" checkbox in the Server Settings tab of Account Preferences.

All of these accounts were working just fine, and I was able to send email at 6pm. No updates took place in the meantime. But by 8pm, none of them worked. And since the issues are across accounts, it's not at the server end. My iPad sends mail just fine on those same accounts. I have no idea what broke.

Receiving email through IMAP is working fine. No updates are pending in the Apple Store. The email accounts are setup "Automatically manage connection settings", turning it off & turning off SSL/TLS fails "Unable to verify account name or password"... though they are obviously right because IMAP can download just fine.

I see similar issues reported here ~4 months ago. No resolution is reported but that Apple was "working on it".

I'm not sure where to go from here.