There's no way in general to check that. Only the mail server at the final endpoint can know for sure if an email address exists. You can't tell just from the email address where that endpoint server is, and even if you could you may not be able to connect to it to ask.
Steve.
PS. This really isn't a SAPgui question and so should have been posted in some other space, although I'm not sure which. It isn't really an SAP question at all...