You're dealing with a mail host outside your own control. There's a good chance you're failing to communicate with it or you're getting a failure response from it.

Take a look at Net::Cmd and in particular the code and message methods to see what the error might be.

A common error when sending email through an SMTP outside your own network is that your ISP will firewall all port 25 traffic to any server other than their own SMTP servers they supply to you. This is a spam prevention measure, and depending on the size (small), the type of your connection (MAC authenticated with custom filtering possible), and service level (high) of your ISP, you might (but don't count on it) have a chance to get port 25 opened up for you by asking and promising not to send spam.