coopster - 8:05 pm on Jan 5, 2009 (gmt 0)

Blacklists were only the first suggestion, somewhat of a quick peek type suggestion to see if there was anything blatantly obvious. You should log into your server which is sending the messages and initiate a TELNET session.

Check your send mail queue to see if the messages are loading up (not being delivered). If the accepting mail server (hotmail) didn't issue your sending mail server a failure notice they should be in the queue for a period of time, typically about seven days ... or perhaps your mail server is blocked. Hotmail may be initiating a connection closure as soon as the connection is made. This emulated behavior is usually indicative that the mail server is busy and you should retry. So basically, their mail server is blocking your mail server because it doesn't meet it's criteria for connecting. All mail servers should identify themselves with a valid hostname with a forward address. The forward dns address must also match the reverse address. Make sure you have your PTR set up correctly.

Here is what an example failure may look like (substitute localhost IP here with hotmail server IP that you are attempting to connect) ...

# telnet 127.0.0.1 25 
Trying 127.0.0.1... 
Connected to 127.0.0.1. Escape character is '^]'. 
Connection closed by foreign host