Doing some further header parsing, I think it is the missing Return-Path header that is tripping up my FUDforum.
Looking deeper I see this (wsp04 is my only theme I am using):
include/theme/wsp04/iemail.inc: $header = 'From: '. $from ."\nErrors-To: ". $from ."\nReturn-Path: ". $from ."\nX-Mailer: FUDforum v". $GLOBALS['FORUM_VERSION']. $extra_header. $header
So Return-Path should be included and be between Errors-To and X-Mailer.
But a sample of headers my FUDforum is sending sees it missing.
Received:
Received:
X-Spam-Checker-Version:
X-Spam-Level:
X-Spam-Status:
Received:
Subject:
Date: Wed, 05 Jun 2013 10:34:
To:
From:
X-Mailer:
From:
Errors-To:
X-Mailer:
MIME-Version:
Content-Type:
Content-Transfer-Encoding:
Message-Id: