I was going to add my vote to Richard Welty'a email (Block relay, Include IP),
but on deepet thought, my vote goes for the default being
Block relay
Include HELO string, IP address *AND* reverse lookup in Received:
with comments nearby explaining what each line does, why, when it may be useful
to change it, to what it may be changed, and pointers into spec.txt to read up
on the matter.
It's the small site, without a sys admin (or just a newbie) at which the
*defaults* should be aimed.
A serious ISP will RTFM and analyse the options (OK -- at least some of them)
before going live.
Most "relaying" (email coming in over SMTP and going back out over SMTP) is
likley to involve one the other MTAs being "local".
Reverse lookup isn't THAT expensive -- especially if exim is invoked from inetd
under tcpd ...
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