How to know if a email address is invalid?

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 开发者_C百科a FB, I get this message:

"Our systems have detected that xxxxxxx@email.it is no longer a valid email. "

So my question is:

How these systems can detect if an email is valid ?

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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.

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You can't reliably determine if an email is valid or not. SMTP servers are suppose to implement the VRFY command but many have this disabled to prevent spammers from being able to detect valid email address.

Additionally, even if an email address respects the RFC 5322 valid email address format does not mean that it is a valid email address on the destination server; many servers do not correctly implement this standard!

The best that you can do is send an email to the address with a click back link in the email. If the email address is valid and the user reads the email and the user clicks the link then it might be the case that the email address is valid (it could have been intercepted too thus giving you a false positive).

If the email bounces back, it might indicate that the email address is not valid (but it could also be the case that the destination server is down, or the mailbox is full, or who knows what else).

As you can see, SMTP is remarkably unreliable given how well it works in practice.

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Most likely Facebook tried to send mail to that address. They know that the address isn't valid because the mail bounced.

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Perhaps they tried to send an email (such as an update EULA) to said host and it responded in an unkind fashion -- hence "not valid"

SMTP is sort of stupid on how it handles -- or doesn't handle -- "can't deliver" messages uniformly, but the various detections may work "well enough" (VRFY, inspecting automatic reply messages, etc.)

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I am pretty sure that FB had sent you any kind of email , and since that your mail is deleted or deactivated ..... there for your email will bounce back to them with the error message like :

No such user 
mailbox unavailable 
etc  ..

so they were able to know that and deactivate your account

if you curious to know that error message just send an email and wait for it to bounce back to you

simple but handy :)

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What some companies do (and possibly FB as well, though I don't know for certain) is periodically send an email to the account, simply to see if it is "live" - they will request the receiver to click a link on it to prove the email address is still in use.

After several such emails that have received no response (or were bounced), they will consider the email address "dead".