On 3 Jan 2018, at 15:42, @lbutlr wrote:
[...]
On 03 Jan 2018, at 12:36, Bill Cole
<sausers-20150...@billmail.scconsult.com> wrote:
About 1.5% of my personal non-spam email over the past 20 years has
had "localhost" as the right hand side of the MID. This implies a de
facto RFC violation because it poses a real risk of duplication.
There is no requirement that the right side be globally unique, just
that the entire message ID is globally unique.
Right. And any software that can use localhost (or any other unqualified
name whose meaning is contextually variable) as the right hand side is
likely to be doing so on multiple machines that don't know about each
other and so generally cannot know that they are not generating
duplicate MIDs. The reason for the RHS=FQDN tradition is to establish a
namespace for each domain whereby global uniqueness can be guaranteed
deterministically.
An additional ~1% has a MID header with either no dots or no '@'.
Dots are irrelevant, but the way I read the RFC, ‘@‘ is required.
See the message I was responding too, which asked about the feasibility
of enforcing a "valid domain" rule. For that, dots are absolutely
relevant. My point, in short, is that doing so may result in 2 orders of
magnitude more rejection of wanted mail than most sites would deem
tolerable.
