This is from the manual but I don't know how to take advantage of it:

Our SMTP servers are configured as secure relays. This means that you cannot simply reference "mail.yourdomain.com" as an Outgoing mailserver unless you successfully log in via one of your pop3 accounts at "mail.yourdomain.com' before you try to send. This is a mandatory setting to prevent spammers from using our mailservers as havens for unsolicited email. If you are getting a "relaying prohibited" or "disconnected by administrator" error, it means that you haven't logged into the pop3 server at your domain before you tried to send through the smtp server at your domain. To log in, you need to check for mail first.


> 
> From: Lennon Day-Reynolds <rcoder / gmail.com>
> Date: 2004/06/18 Fri PM 05:55:43 GMT
> To: ruby-talk / ruby-lang.org (ruby-talk ML)
> Subject: Re: Net::SMTPFatalError: 550 Administrative prohibition
> 
> I believe that David was trying to establish whether the code was
> running as root or not, since only root can open server ports lower in
> number than 25. To connect as a client, though, you need no special
> privileges, and any effective user ID is fine.
> 
> Since you're using the Net::SMTP library, you're obviously connecting
> as a client to the mailhost, so the port number won't be an issue. In
> fact, since the error you're seeing is one of the standard SMTP codes,
> you've already connected to the server and begun a session when it
> happens.
> 
> My guess is that you're being blocked from sending mail due to the
> security and/or anti-spam policies configured into your ISP's SMTP
> server. My guess is that something about the format of your message is
> triggering a spam filter -- i.e., connecting from an IP address
> outside the ISP's netblock, having mismatched senders in the message
> headers and sendmail call, or something similar.
> 


> alternately, you could just contact your ISP and ask someone what
> their policy is on scripting mail transmission. 
> > What is a restricted account?
> > 
> > >
> > > From: David Ross <dross / code-exec.net>
> > > Date: 2004/06/18 Fri AM 04:25:19 GMT
> > > To: ruby-talk / ruby-lang.org (ruby-talk ML)
> > > Subject: Re: Net::SMTPFatalError: 550 Administrative prohibition
> > 
> > 
> > >
> > > >
> > > > Net::SMTP.start(
> > > >    'mail.myServer.com',
> > > >    25,
> > >
> > > What OS are you using? Try a port beyond 1024.
> > > Are you running that code in a restricted account?
> > >
> > > --David Ross
> > >
> > > >    'mail.myServer.com',
> > > >    userName,
> > > >    password,
> > > >
> > > >    :login) do |smtp|
> > > >
> > > >         smtp.sendmail( msg,  "from / me.com", "to / you.com" )
> > > >       end
> > >
> > >
> > 
> >
> 
>