How do I verify whether someone's ymail add...

Many mail servers will immediately send you a delivery failure notification if the user is not valid (like the mailbox doesn't exist). If you send them a test message and you get a failure you have your answer. You could also try calling the person you are trying to add to Facebook (assuming you have his/her phone #) to verify the e-mail address.

However, Facebook does a nice job of searching by e-mail address. In Facebook, click on Friends from the top menu, click on Find Friends from the left side navigator, scroll to the bottom and type in the e-mail address in the Search for People box. Click the maginfying glass icon. If the owner of that e-mail address has a Facebook account it will show up, if not you will be givent the opportunity to e-mail them.

You can also try http://verify-email.org/ - I found this on a Google search. Google is everyone's friend. :-)