and see if that may apply, particularly the solution offered (by yoz). It's not the complete story as vanilla Yahoo is different to BTYahoo, which I suspect your account is. Webmail will show a Yahoo image if it is.
Some of the other replies on that thread may not really apply - like only sending to 5 at a time, or signing up to forums etc. If that's the case Yahoo (and therefore possibly BTYahoo) should never be used again. Granted the posts are almost 2 years old and things may have moved on.