title: verify email address on Linux
body: <p>I am using command like <code>sendmail -bv example@othesite.com</code> to verify some email address to confirm the address is  valid or not before sending actual email to people.
  it works for most sites I tested. But sometimes, I am using the sendmail
  for some website that will give me all pass no matter what the address name I put for
  the same site.</p>
  
  <p>For example:</p>
  
  <p><code>sendmail -bv yyyyyy@othersite.com</code></p>
  
  <p>where <code>yyyyyy</code> can be anything that also passes the command and always reported out  "<code>yyyyyy@othersite.com</code>  verified". It seems correct but I think that should be something wrong behind.  Why ?</p>
  
  <p>Other command checking mail address on linux will be reliable and available, how and what command does this?</p>