What traits make a junk email filter classify email as junk?

I AM NOT TRYING TO SEND SPAM.

That said, one of my customers is going through a merger and acquisition.  As part of the process, they (meaning me) must email all their customers (50,000 or so) with a notice that they are being acquired and offering to remove anyone's name from the database before the acquisition.

I want to avoid my email from being classified as junk (actually I'd prefer everyone just ignore it so I don't have to process as many opt-outs, but gotta do the right thing).  I sent a test message to my spare hotmail account and it landed in junk, but I don't know why.  The gist of the email

From: PrivacyPolicy@MyClientCompanyInfo.com (this is a domain I control so I have set myself up as a good SPF mailer)
Subject: Notice regarding change of ownership of <my customers name>

The body is about one screen long along the lines of "We are being bought.  We got your email address because you are one of our customers.  We don't sell email addresses, but we are being bought, remember? so if you don't want the new guys to get your email address, reply with Unsubscribe".  It has one link to the web sites privacy policy.  I suspect some of the verbage, like "Simply reply to this email with the word "Unsubscribe" in the subject line to have your information removed from our database" might trigger junk filters, but I don't know what the key bad words are.

Anyone have some suggestions on what I should avoid?

Did I mention I am NOT SENDING SPAM
LVL 16
JohnBPriceAsked:
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PsiCopConnect With a Mentor Commented:
The answer to your question is: Yes.

There is no single standard as to what will cause a SPAM filter to label or reject an E-Mail. Every anti-SPAM system has its own rulesets, and every mail system admin has their own configuration.

General things to help avoid having your E-Mail classified as SPAM:

0) DO make sure your E-Mail server(s) are properly registered in DNS, including appropriate rDNS records. Having an SPF record may help, as well.

1) DO NOT send HTML E-Mail.

2) DO make sure your E-Mail server(s) are not open relays.

3) DO NOT send E-Mail with lots of attachments, especially binary or image attachments.

4) DO make sure that your E-Mail conforms to standards. If you generate E-Mail with MIME parts, make sure your MIME structure is valid. Make sure your message headers are valid and sane.

5) DO make sure your mail system conforms to standards, including accepting E-mail for "postmaster" and "abuse" aliases, waiting for the HELO banner, and honoring return codes (e.g. requeuing for a reasonable period when getting a TEMPFAIL, ditching the message recipient when getting a PERMFAIL).

6) DO NOT send HTML E-Mail.

I would suggest that you operate with an opt-in policy, as opposed to the opt-out approach you are taking. Your "Reply with unsubscribe" text will, as you suspect, twitch the antennae of many anti-SPAM systems. How about an approach where the possibly former customer will *not* receive any more E-Mails *unless* they go sign back up for them?
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JohnBPriceAuthor Commented:
Thanks PsiCop, just the kind of stuff I was looking for.  My rDNS for the domain will not resolve to the mail server I am sending from (because I don't want to hose up my mail server), but the SPF does specifically include the mail server I am sending from.

>>I would suggest that you operate with an opt-in policy, as opposed to the opt-out approach you are taking.
Alas, out of my control, we are talking fortune 500 with fancy pants lawyers.
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JohnBPriceAuthor Commented:
>>fancy pants lawyers.
I can change the verbage if I know what to change it too and get it approved by the lawyer.
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Rich RumbleSecurity SamuraiCommented:
Spam filters trigger on various filters an indicators, such as the email being just a .gif or .jpg. bayesian filters and markovian discrimination do the majority of the filtering, as well as RBL's and whitelist's. http://en.wikipedia.org/wiki/Bayesian_filtering#Process
http://en.wikipedia.org/wiki/Markovian_discrimination
As pointed out above, HTML email is a big no no, as well as making sure you don't get RBL'd http://en.wikipedia.org/wiki/DNSBL
-rich
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JohnBPriceAuthor Commented:
OK, I fooled with the wording and got my junk rating down from medium to low, but clearly the issue is that my sending server is not my main mailserver for that domain.  If I send the original text from the main mail server, it does not get labeled as junk.  Not sure why.  Here are redacted versions of both mail headers.

not junk
X-Message-Status: n:0
X-SID-PRA: PrivacyPolicy@<MyClient>.com
X-SID-Result: TempError
X-Message-Info: LsUYwwHHNt1Z3XYBI0K3HyQqWkUJ7U6RkIO7o7rYDoU=
Received: from gwa3.webcontrolcenter.com ([63.134.207.12]) by bay0-mc6-f2.bay0.hotmail.com with Microsoft SMTPSVC(6.0.3790.2444);
       Fri, 27 Oct 2006 04:47:03 -0700
Received: from maila10.webcontrolcenter.com [216.119.106.127] by gwa3.webcontrolcenter.com with SMTP;
   Fri, 27 Oct 2006 04:46:42 -0700
Received: from 29.bktr8.xdsl.nauticom.net [66.37.61.126] by maila10.webcontrolcenter.com with SMTP;
   Fri, 27 Oct 2006 04:46:06 -0700
MIME-Version: 1.0
Date: Fri, 27 Oct 2006 07:46:05 -0400
Message-ID: <CHILKAT-MID-2790c77c-6542-4123-a56d-f759bf6fe55e@John.databanque.com>
X-Mailer: Chilkat Software Inc (http://www.chilkatsoft.com)
X-Priority: 3 (Normal)
To: "John Price" <<MyHotmail>@MSN.com>
Subject: Notice regarding change of ownership of <My clients company> from m.db.com
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
From: PrivacyPolicy@<MyClient>.com
Return-Path: privacypolicy@<MyClient>.com
X-OriginalArrivalTime: 27 Oct 2006 11:47:03.0408 (UTC) FILETIME=[9EBED700:01C6F9BD]


Junk
X-Message-Status: n:0
X-SID-PRA: PrivacyPolicy@<MyClient>.com
X-SID-Result: TempError
X-Message-Info: txF49lGdW41x8fFGA0sfVNsj+U9cTYkmtERoexhiRsE=
Received: from dellserver.databanque.com ([66.37.61.126]) by bay0-mc2-f1.bay0.hotmail.com with Microsoft SMTPSVC(6.0.3790.2444);
       Fri, 27 Oct 2006 05:21:22 -0700
Received: from dellserver.databanque.com ([192.168.1.101]) by dellserver.databanque.com with Microsoft SMTPSVC(6.0.3790.211);
       Fri, 27 Oct 2006 08:21:01 -0400
MIME-Version: 1.0
Date: Fri, 27 Oct 2006 08:21:01 -0400
Message-ID: <CHILKAT-MID-f7c63a8d-8985-4fed-be65-559052f00006@dellserver.databanque.com>
X-Mailer: Chilkat Software Inc (http://www.chilkatsoft.com)
X-Priority: 3 (Normal)
To: "John Price" <<MyHotmail>@MSN.com>
Subject: Notice regarding change of ownership of <My clients company> from dellserver
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
From: PrivacyPolicy@<MyClient>.com
Return-Path: PrivacyPolicy@<MyClient>.com
X-OriginalArrivalTime: 27 Oct 2006 12:21:01.0401 (UTC) FILETIME=[5D7BE490:01C6F9C2]

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PsiCopCommented:
"192.168.1.101"

If I had to guess...and I do...I'd say that's the problem. The receiving mailserver is examining the "Received: from" headers and noting a non-routable/reserved IP address.
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JohnBPriceAuthor Commented:
Yea, that is the internal address of my exchange server.  Lots of people must do that, though, so I thought receivers might examine the chain and find that 66.37.61.126 is a backup MX address for the domain.  Guess not though.  I wish I could figure it out, but for now I am going to bail and send them from my main mail server for the domain.  Thanks for the tips.
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