Yahoo mail wouldn't send. Solved!
While on a support call to see why my Yahoo mail wouldn't send, I spotted the problem.
g-mail worked and I could even receive on Yahoo, but send, just wouldn't.
Long story short, it was Expat Shield. For some reason it's taken to turning itself on. Lots of times.
Using IE made not a jot of difference. Buy why just Send, and only on Yahoo?