Can you not use a 'comcast.net' email address solely for the authentication? SBC put their customers through this same wringer ten years ago. What I did was to configure my client to use '******@pacbell.net' for authentication, but '********@aosake.net' for the "From:" address. Nobody ever saw the 'pacbell.net' address, just the 'aosake.net' address.
Addendum:
As an example, here are headers (my server is temporarily offline, so I used another account):
x-store-info:J++/JTCzmObr++wNraA4Pa4f5Xd6uens/GgAwQp04qJrhQ4X0uoZBxy/dkHL
xXCBwcjWNRRSpd5wSmJIm7sPnF90XvpI8tgDdQqFgQ94hQrwykr8H4IG9Nepp4wYI/bbM9fSYt1l+Dw=
Authentication-Results: hotmail.com; spf=none (sender IP is 64.142.19.5)
smtp.mailfrom=**********@netscape.net; dkim=none header.d=netscape.net; x-hmca=none
X-SID-PRA: **********@netscape.net
X-AUTH-Result: NONE
X-SID-Result: NONE
X-Message-Status: n:n
X-Message-Delivery: Vj0xLjE7dXM9MDtsPTE7YT0xO0Q9MTtHRD0xO1NDTD0w
X-Message-Info: aKlYzGSc+LngSIEN9nC7GRaVlXLN7zbKfIbKa3LQPpvRklgyf+bf1scoommzxBTAkAMxYwudjV
0qga6n0nCDcfOtPllc5DBdQN2m8xxSK/yMpJH5Hzv/zxqZOrK/sk+sVaTixt5quAZd91kfuAq5n0OMMoo45RWa
Received: from b.mail.sonic.net ([64.142.19.5])
by COL0-MC4-F26.Col0.hotmail.com with Microsoft SMTPSVC(6.0.3790.4900);
Wed, 10 Apr 2013 22:50:16 -0700
Received: from akari.akari.aosake.net (reki.aosake.net [173.228.7.217])
(authenticated bits=0)
by b.mail.sonic.net (8.13.8.Beta0-Sonic/8.13.7) with ESMTP id r3B5oFIN030506
(version=TLSv1/SSLv3 cipher=AES256-SHA bits=256 verify=NO)
for <**********@hotmail.com>; Wed, 10 Apr 2013 22:50:15 -0700
Content-Type: text/plain; charset=iso-8859-15; format=flowed; delsp=yes
Date: Wed, 10 Apr 2013 22:50:14 -0700
To: "**********@hotmail.com" <**********@hotmail.com>
Subject: [TEST] SMTP Server
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
From: "NormanS" <**********@netscape.net>
Message-ID: <op.wvc3tza65ghexu@akari.akari.aosake.net>
User-Agent: Opera Mail/12.15 (Win64)
Return-Path: **********@netscape.net
X-OriginalArrivalTime: 11 Apr 2013 05:50:16.0527 (UTC) FILETIME=[714E85F0:01CE3678]
Lines 15 through 19 show my ISP (Sonic.net) message submission server accepting email with authentication.
Line 26 shows the From: address as a Netscape domain.
No line shows my Sonic.net email address, though it was needed to authenticate to the server.
This is pretty much what NetFixer
showed in his headers. When I was with AT&T, this still worked the same; different servers, same results.
Comcast expects you to authenticate to their message submission server with a Comcast UserID; they don't require From: to be a Comcast email address.