Network Programming in .NET

One of these two email addresses is valid : gianluca.91@gmail.com or gianluca.92@gmail.com – how can you tell which one? Regexes will say both are valid, even a DNS MX lookup will say that @gmail.com is valid.

Here’s the trick: https://avatarapi.com/avatar.asmx?op=VerifyEmail

It’s a free API, that does not require registration, or authentication, and does not store the email addresses supplied to it. It does not send an email, but just checks the mailbox.

Here is a result for gianluca.92@gmail.com

<EmailVerificationResponse xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://avatarapi.com/">
<Verification>FAIL</Verification>
<MailExchange>alt4.gmail-smtp-in.l.google.com.</MailExchange>
<SmtpResponse>550-5.1.1 The email account that you tried to reach does not exist. Please try</SmtpResponse>
</EmailVerificationResponse>

And here is the result for gianluca.91@gmail.com

<EmailVerificationResponse xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://avatarapi.com/">
<Verification>SUCCESS</Verification>
<MailExchange>alt4.gmail-smtp-in.l.google.com.</MailExchange>
<SmtpResponse>250 2.1.5 OK hf21-20020a17090aff9500b001bc3052777csi2002522pjb.42 - gsmtp</SmtpResponse>
</EmailVerificationResponse>

It also works with every email host, not just Gmail. However, some mail exchangers do not give information on their mailboxes, in which case the result can be inconclusive.