Ok this is somenthing unbelievable, we have a website, users sign up and receives links to confirm they signed up BUT: 1 - microsoft blocked our IP (no one with microsoft email account can receive our emails) 2 - we tryed contacting microsoft submitting the detailed form about our problem 3 - we posted 3 times in their community about our problem 4 - we tweeted they about our problem 5 - we tryed finding out some telephone support number (the few there are arent' helping at all) Do you think we solved? the answer is NO :/ We still unable to send emails from our IP to microsoft email accounts, since 3 months back. Our emails are perfect we checked all the email headers following microsoft guidelines but it seems not enought, checking our IP reputation it seems everythings ok, indeed we can send email easly to any other provider , gmail, yahoo, etc Do you know any other way to try to get help ? FULL ERROR RETURNED BY MICROSOFT: host mx1.hotmail.com[65.55.37.120] said: 550 SC-001 (COL0-MC4-F28) Unfortunately, messages from 94.23.***** weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. (in reply to MAIL FROM command) We are running NGIX + php mailer from a Virtual Private Server (No Hosting or shared hosting)