> I've been looking through the documentation and > the 9fans archive but I can't get a clear answer on > what to replace localhost.localdomain with.
If the recipient's mail server is being strict (but within the bounds of the RFCs), that name is expected to be the real, externally-resolvable DNS name of the system you're sending from. The RFCs used to be more lax on that point, and some servers still are, but you shouldn't assume you'll be able to send to arbitrary endpoints unless you satisfy that.
