[Exim] localhost not local?

I am just setting up exim for the first time.

exim did not recognize localhost as a local domain until I added
localhost to local_domains in exim.conf. Now it works fine.

My question is, is this wrong?

The fact that localhost is not in local_domains in the default exim.conf
or written by default by eximconfig makes me think I may be doing
something wrong.

fetchmail release 5.5.0+IMAP-GSS+SSL
exim 3.22-10