EmailVerify.NET

Greylisting (or graylisting) is a simple technology aimed at defending mailboxes against spam; used by many email servers, greylisting tries to defer the acceptance of an email message by some minutes by having the target mail exchanger returning a temporarily reject, in the hope that a potential spammer would not retry the transfer because of bandwidth and timing restrictions.

EmailVerify.NET detects a greylisting condition automatically and aborts the validation process with a specific failure; it is up to the implementor to choose if retrying the validation after some time or not.

The following code mimics a possible email validation program that takes into account a possible greylisting condition:

            var verifier = new EmailVerifier();

            // ...

            var result = verifier.Verify("alice@example.com",
                VerificationLevel.Mailbox);

            if (result.IsSuccess)
            {
                // The mailbox exists
            }
            else
            {
                switch (result.Status)
                {
                    case VerificationStatus.MailboxTemporarilyUnavailable:
                        // Possible greylisting condition, needs to queue
                        // this validation again, in a couple of minutes

                        // ...

                        break;

                    // TODO: Add more cases, if needed

                    default:
                        // ...
                        break;
                }
            }