Email address validation tool - recommendations/experience? [closed]

"You can not reliably check that a particular address is valid because many mail servers will not tell you directly during the SMTP conversation. Many are explicitly configured not to respond with ""user/mailbox does not exist"" errors in order to stop outsiders being able to derive a list valid addresses. Also the SMTP server you connect to may just be a relay that forwards messages on to servers deeper in the network - in this case the server you are talking to may not know at all what addresses are valid, it will just what domains to handle and where to relay the mail for them (so it will accept any address for those domains, and something else will later in the process will throw out the invalid ones).

Even actually sending a message would not work unless you wait for the user to confirm receipt of the message, as not all servers send bounce messages for invalid accounts.

Also be careful when checking for invalid domains as part of your validation that you account or temporary DNS problems and other such intermittent network glitches, otherwise you could throw out addresses that are perfectly valid under normal conditions."