Dear email users.
Read the bounce backs, the answer is ALWAYS THERE!
Customer: "I can't send to email@example.com"
Me: "OK, forgetting for the minute that that's not my problem and you should be speaking to Yahoo, send me the bounce back and I'll have a look."
A few minutes later.
Bounce Back: "This user doesn't have a yahoo.co.uk account"
That'll be the reason why then? Sheesh.