Any objections to rejecting email based on sender callout check?

[Ian Kelling]

We've been doing sender callout checks, but failure only caused a 20
minute delay through greylisting. JohnS suspects we were rejecting based
on this in the past and there's a whitelist in our exim config for some
people who had bad email servers. I've had it on for my mail server for
a year or so without any problem.

reference: 
https://www.exim.org/exim-html-current/doc/html/spec_html/ch-access_control_lists.html#SECTcallver

-- 
Ian Kelling | Senior Systems Administrator, Free Software Foundation
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