Actually, with the changes that I described, my email has been sending perfectly for more than 24 hours. To repeat, I made the following changes:
1. I added my ISP's two DNS servers to the one that was already in the Network settings of System Preferences. For whatever reason, when I ran my ISP's installation program, those servers were not added. The one and only DNS server - before I added the two - was outside my ISP.
2. For my main account, I replaced "smtp.att.yahoo.com" with "68.142.198.11"; for my wife's subaccount, I left the "smtp.att.yahoo.com" alone.
You can run "whois smtp.att.yahoo.com" at a terminal command line, with the appropriate change for your smtp server, to get the numeric address.
I spent two hours on the phone with level 1 & 2 support of my ISP, and another one hour on the phone with Apple tech support, and no one thought to have me try these changes. The people who do this for a living should have figured this out before a dumb lawyer. Then again, they do not have my wife complaining about her email not working. 😉