IP Address Blocked for SMTP, Server or Account IP?

[Al Franz]

Thought someone in the BIND group would be best to answer my question.

My ISP server I have my account on is not delivering sent mail to msn.com or 
hotmail.com email addresses.  This server has an IP address and I have an 
account that has it's own IP address on this server.  Which IP address is 
Microsoft blocking most likely?  The IP address of the server or does SMTP 
run under each individual accounts IP address if they have one?