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Date: Sat, 20 Jun 2015 23:12:19 -0700
From: Gregory Shapiro <gshapiro@gshapiro.net>
To: Jamie Landeg-Jones <jamie@dyslexicfish.net>
Cc: freebsd-stable@freebsd.org
Subject: Re: Last openssl update brakes localhost email sending
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> I'm curious... Why is localhost delivery encrypted by default in the first place?

sendmail, when acting as a client, employs opportunistic encryption by default.  Local mail submission done via command line uses the MSP configuration /etc/mail/submit.cf to send the mail.  That submit.cf is built to relay the mail to an MTA host, localhost by default, but can be configured to use a central mail server as well if desired (e.g., for a centralized mail hub, centralized queue management, etc.).  The standard submit.cf makes no assumptions about the location of the MTA host and therefore doesn't disable encryption.

> The only reason I can think of is if there is some unencrypted TCP
> relayed 'tunnel', that has been set up not using ssh or some other
> encrypted transport.

One other use case (likely not a concern) is to prevent other privileged users from easily snooping localhost traffic (`tcpdump -i lo0 -X -s 0 port 25`).