This test has always been unpopular, even back when you got a mailing loop
within 24h if you disabled it ;-) We decided to disable it by default in
14.5, because most people have fixed their mail software and the kinds of
loops this test was designed to prevent no longer occur on a typical
discussion list. But on the other hand, there are still exceptions, and this
test is the only reliable way to catch these kinds of loops. To put things
in perspective, whenever you post something to a discussion list and you,
the poster, receive a notice that mailbox such and such does not exist, it
is a signal that the list may be at risk for this type of loop. Nowadays,
this almost never happens, but there are still lists where it does. The
larger the list, the higher the risk. If you don't want to lose this test
when 14.5 is installed on your server, you can add "Loopcheck= Full" to your
list header now.

The fundamental problem that this test addresses is that Internet mail
standards define 5 different origin headers for an e-mail message (that's
not even counting 'Errors-To:' and other unofficial headers), whereas most
corporate/intranet/homebrew mail systems only support one origin. When
gatewaying from the Internet to the corporate mail system, one origin header
has to be picked, and the rest are lost in transit. And this one address is
where bounces will go. Most gateways put the list address in the internal
origin field, some use the poster's address. Either way, they almost never
pick the bounce return address for the message origin, so bounces go either
to the list or to the poster, not to the owner-xxx address. Since these
systems typically also don't follow Internet standards for bounce formats,
you have a nice loop if the bounce goes to the list. The '"Sender:", "From:"
or "Reply-To:" field pointing to the list' test is very effective at
blocking these loops, but it also blocks replies from some types of mail
programs.

  Eric