no special magic should be needed, BUT...
(1) if sendmail is getting the hostname for the ibook
from someplace dynamic, like dhcp, weird things
could well happen. you might want to hardcode the
name into the sendmail.cf by using something like
in the .mc file. (the .mc file is used to make the .cf file, as
per the article).
(2) the network where you have plugged in the ibook might
not let just any box there send mail out. For some places,
you'll probably have to use the ISP or corporate smarthost.
Be aware that this also means that you could try to send
some mail, not notice that it didn't go out, and then have it
go out days later when you plug the laptop in someplace else.
(3) make darned sure that the machine is emitting headers which can be replied to. not to do so is irresponsible and
can cause other folks lots of hassles.
you might want to have mail for laptop.domain.com received on a regular basis by some machine which does not move around to different networks, and which would then deliver/forward the mail someplace where you'll get it.