How to know if a email address is invalid?

Question

My email address on www.email.it has been disabled because I haven't used it for a long time.

Now, when I go on a FB, I get this message:

"Our systems have detected that [email protected] is no longer a valid email. "

So my question is:

How these systems can detect if an email is valid ?

Answer 1

You cannot detect if an email is fake. All you can do is to detect whether a string respects the RFC 5322 of a valid email address format. You can't possibly know whether such an account has been declared at the target SMTP server unless you own this server. You can probably try to send an email to this address and see what does the remote SMTP server responds.