Thought you'd like to know.
To quote
http://projects.puremagic.com/greylisting/whitepaper.html?view=markup:
"One issue is that some MTA software (Exim for example) attempts to
limit the problem of forged sender addresses by attempting to verify
that the claimed sender of an email is a valid address by doing an SMTP
callback before accepting mail. Since it is desired to minimize the
traffic when a mail may be rejected temporarily, the best course of
action would be to issue a tempfail after the RCPT command. However, in
the case of a SMTP callback, doing so at that point may cause our
outgoing mail to be delayed unnecessarily."
I don't think exim does that by default (anymore?). It could get you
blacklisted someone wrote here recently.
Regards,
Jeroen
