What does "service not available" mean in Perl's Mail::Sender?

I'm trying to send mails from Perl using the Mail::sender module. But I keep getting -4 return messa开发者_开发技巧ges. The manual says -4 is service not available. What does that mean?

"Service not available" generally means that the server you are trying to contact is not online. In your case it is probably the SMTP-server that is offline.