validation - Email SMTP validator

Question

I need to send hundreds of newsletters, but would like to check first if email exists on server. It's called SMTP validation, at least I think so, based on my research on Internet.

There's several libraries that can do that, and also a page with open-source code in ASP Classic (http://www.coveryourasp.com/ValidateEmail.asp#Result3), but I have hard time reading ASP Classic, and it seems that it uses some third-party library...

Is there some code for SMTP validation in C#, and/or general explanation of how it works?

Answer 1

Be aware that most MTAs (Mail Transfer Agent) will have the VRFY command turned off for spam protection reasons, they'll probably even block you if you try several RCPT TO in a row (see http://www.spamresource.com/2007/01/whatever-happened-to-vrfy.html). So even if you find a library to do that verification, it won't be worth a lot. Ishmaeel is right, the only way to really find out, is sending an email and see if it bounces or not.

@Hrvoje: Yes, I'm suggesting you monitor rejected emails. BUT: not all the bounced mails should automatically end up on your "does not exist"-list, you also have to differentiate between temporary (e.g. mailbox full) and permanent errors.