Setting hostname - fake and real

[Ben Paley]

Hello,

Is there an easy way to set a fake hostname in rc.conf which will let sendmail 
send mail to external domains?

For ages I've had the line in rc.conf

hostname="potato.fake_domain.net"

(not exactly, but you see what I mean...) and I've had no problems with it at 
all. Recently I've tried to set up sendmail, only so I can test php or perl 
scripts that send emails!

Sending to localhost works fine, but mail to an external address is returned 
by the remote domain because it couldn't verify my domain. Very sensible, I 
could be a spammer. So I edited rc.conf to say

hostname="potato.real_domain.net"

that is, I used the name of a real domain which is registered to me. The 
domain is hosted on the net, but AFAIK they don't have a machine called 
"potato". Now apache won't start and I get weird errors in other things, like 
shutting down X even!

Can anyone help?

Thanks very much,
Ben