Peter,

... so it confirms the earlier experiments and points to the culprit
being btinternet or synchronoss.

And, following up with a similar case reported by another member I have evidence to implicate synchronoss.net
https://groups.io/g/GroupManagersForum/topic/33141231

Off-list Trista sent me the header for a misdirected message that posted into her group. In the following two Received fields note particularly the "for" clauses:

Received: from re-prd-rgout-002.btmx-prd.synchronoss.net ([10.2.54.5])
by re-prd-fep-041.btinternet.com
with ESMTP id

<20190904180319.WRUD7208.re-prd-fep-041.btinternet.com@re-prd-rgout-002.btmx-prd.synchronoss.net>

for <ta-members@groups.io>; Wed, 4 Sep 2019 19:03:19 +0100
...
Received: from [redacted]-imac.home ([redacted)
by re-prd-rgout-002.btmx-prd.synchronoss.net (5.8.337)

(authenticated as [redacted]@btinternet.com)

id 5D6FA3290010097D
for ta-members+owner@groups.io; Wed, 4 Sep 2019 19:03:19 +0100

The second is synchronoss saying that they received the message from the member's iMac, intended for the +owner address.

The first is btinternet saying that they received the message from synchronoss, intended for the group posting address.

I don't know for a fact about these two services, but I believe that servers generally record in the Received header very literally what they received. Which in this case would imply that synchronoss trimed the "+owner" off of the address after receiving it from the member's iMac and before delivering it to btinternet.

That's what I call a smoking gun.

Shal


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