I think noone has answered because this appears to be an incredibly simple question. If there is a bigger problem here, please explain. A valid email fits the format of email@example.com. You check to see if it exists by looking at it. If you want more information, you'll have to give us more details about your scenario. Also, please don't ask people to respond ASAP. Usually people will ignore posts that request that.
Email Width should be around 64-70 characters
It should accpet Alphabets, Numbers, @ (Only one instance), Hyphen (more than one instance), Underscore (More than One instances), and Period (More than One instances)
It Should not start or End with @, Period and an Underscore.
You could do a dns lookup on the bit after the @. For exmaple, php has a function to do this. That will not tell you if the email address exists, only that the domain does. I have seen a regular expression that checks that an email address is well-formed but it was a screenful!