At least in MS Outlook, the outgoing email address will be underlined once recognized as valid by the computer. Usually, the underlining occurs almost instantly. How does the computer know it is a valid email address before it is sent? 
- Jinx
It doesn’t know if it is an actual e-mail address that will get to an actual person/company. All it knows is that it has a valid e-mail address format. You could type in abc@def.ghi and it would underline it.
It be smart! :rolleyes:
Actually, it’s a little more complicated than ski’s explanation. Outlook actually does a look up on what you type to see if it matches any name or nickname from your various address books. Failing a match there, it will underline well-formed (to steal an XML term) email addresses for systems it has installed (like SMTP, conventional Internet email).
it is just one more example of bill gates peeking out at you from behind your monitor. you know he is there, he knows he is there. he is watching every keystroke you make, every teen porn site you download, every email you send.