How to check if an email address exists without sending an email?

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There are two methods you can sometimes use to determine if a recipient actually exists:

  1. You can connect to the server, and issue a VRFY command. Very few servers support this command, but it is intended for exactly this. If the server responds with a 2.0.0 DSN, the user exists.

    VRFY user

  2. You can issue a RCPT, and see if the mail is rejected. 

    MAIL FROM:<>
RCPT TO:<user@domain>

If the user doesn’t exist, you’ll get a 5.1.1 DSN. However, just because the email is not rejected, does not mean the user exists. Some server will silently discard requests like this to prevent enumeration of their users. Other servers cannot verify the user and have to accept the message regardless.

There is also an antispam technique called greylisting, which will cause the server to reject the address initially, expecting a real SMTP server would attempt a re-delivery some time later. This will mess up attempts to validate the address.

Honestly, if you’re attempting to validate an address the best approach is to use a simple regex to block obviously invalid addresses, and then send an actual email with a link back to your system that will validate the email was received. This also ensures that they user entered their actual email, not a slight typo that happens to belong to somebody else.

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