I signed up for msn hot mail now my yahoo email address is no longer valid, how do I fix this problem?

I signed up a few minutes ago and tried to use my yahoo email address and a posting came up saying the address wasn't valid what did I do wrong when I signed up for msn hotmail?

I signed up for msn hot mail now my yahoo email address is no longer valid, how do I fix this problem?
We have both, signing up for HotMail is not the problem. you might have clicked something on sign-up to transfer? Check at both ends.