This probably has to do with how your outgoing SMTP server is defined and what ISP you are using at the time. You should use the outgoing SMTP server name of the ISP you are currently logged in as.

E.g., my email address is me@mycollege.edu

When trying to send mail via me@mycollege.edu on campus I must use mycollege.edu as the outgoing SMTP server, but at home where my ISP is @home.com, I must specify the appropriate home.com outgoing SMTP server name.

Update: This is assuming that both servers are using TCP wrappers that prevent relaying, as do most servers nowadays.

@a=split??,'just lose the ego and get involved!';
for(split??,'afqtw{|~'){print $a[ord($_)-97]}