title: Email address validation services - how do they commercialize an uncertain service?
body: <p>There are numerous paid services these days that claim to provide "complete" email validation (check for MX records, check with the SMTP server etc), like <a href="http://www.towerdata.com/services/web/email_validation.html" rel="nofollow noreferrer">this one</a>, <a href="http://www.strikeiron.com/Catalog/ProductDetail.aspx?pv=5.0.0&amp;pn=Email%20Verification" rel="nofollow noreferrer">this one</a> and <a href="http://www.briteverify.com/pricing" rel="nofollow noreferrer">this one</a>.</p>
  
  <p>From various past threads on SO, it's evident that foolproof validation of email addresses is not possible - </p>
  
  <ul>
  <li><a href="https://stackoverflow.com/questions/27474/email-smtp-validator">Email SMTP validator</a></li>
  <li><a href="https://stackoverflow.com/questions/504631/determining-the-health-validity-of-an-email-address">Determining the health/validity of an email address</a></li>
  <li><a href="https://stackoverflow.com/questions/565504/how-to-check-if-an-email-address-exists-without-sending-an-email">How to check if an email address exists without sending an email?</a></li>
  </ul>
  
  <p>Any such service is liable to come across the following issues - </p>
  
  <ul>
  <li>Most mail servers won't respond correctly to the VRFY command for antispam reasons, so it's not possible to use it.</li>
  <li>The services' will have their IP(s) blacklisted if they try to access mail servers over SMTP too many times, which they have to, to provide such a service</li>
  </ul>
  
  <p>My question is, is there any other way that this can be done? How do these services claim reliable validation in light of the above? I know that they are not going to reveal their methodologies, but can anyone on SO shed some more light?</p>