Please make sure that you are not running a open -relay mail server. Please run the command "grep u...@hotmail.com(correct hotmail-id) /var/log/maillog.
You can see in the log , the mail was delivered to a specific IP. Check that IP details, whether it is an abuse.hotmail.com using some sites like "whois" indnsstuff.com.
I think the problem is , since you are running PHP script to send mails ,the other mail servers may consider them as spam, since these mails will travel with an user identity of 48.