Yahoo still blocking all emails
Note: Yahoo users are still blocked. Anyone using Yahoo email cannot receive emails from this site. Sign up for gmail instead.
After more research it looks like the whole issue of email blocking was a combination of Yahoo and me hitting the flush queue button a tad too often. Hitting the button caused a hiccup that made other ISPs also block this site. Those blocks were quickly removed once the other ISPs were contacted.
Yahoo is a thicker nut to crack. They block the IP range this site is in days later after been told and told. Presumably, I guess, because historically it was the IP of a spam site. Since the total number of IPs is limited and spammers keep changing IPs then it inevitable that most IPs will have a dubious past history if you look back far enough. For those who are wondering what's an IP the best simile is to think of a hotel, Yahoo is blocking messages from an entire floor of the hotel because a spammer once stayed in one of the rooms on that floor some years ago.
Interestingly and unsurprisingly (to me) if you google
You get over 240,000 hits, most of which look to be valid hits and not off topic. Yahoo use their own IP blacklist as this site's IP is not on any other blacklist. Telling Yahoo they have this wrong is a exercise which makes hitting yourself on the head with a brick look like a better idea. They just don't listen.
For anyone with the same problem, a [T03] error, I can say that Yahoo's response of an email suggesting that you need to implement DKIM, SPF and Yahoo CFL is total tosh. With another domain on a different IP outside the blocked range Yahoo emails go through without DKIM or Yahoo CFL and as far as I remember without SPF either.