Spam Filtering for Mail Exchangers:
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Chapter 2. Techniques

Table of Contents

2.1. SMTP Transaction Delays

2.2. DNS Checks

2.2.1. DNS Blacklists
2.2.2. DNS Integrity Check

2.3. SMTP checks

2.3.1. Hello (HELO/EHLO) checks
2.3.2. Sender Address Checks
2.3.3. Recipient Address Checks

2.4. Greylisting

2.4.1. How it works
2.4.2. Greylisting in Multiple Mail Exchangers
2.4.3. Results

2.5. Sender Authorization Schemes

2.5.1. Sender Policy Framework (SPF)
2.5.2. Microsoft Caller-ID for E-Mail
2.5.3. RMX++

2.6. Message data checks

2.6.1. Header checks
2.6.2. Junk Mail Signature Repositories
2.6.3. Binary garbage checks
2.6.4. MIME checks
2.6.5. File Attachment Check
2.6.6. Virus Scanners
2.6.7. Spam Scanners

2.7. Blocking Collateral Spam

2.7.1. Bogus Virus Warning Filter
2.7.2. Publish SPF info for your domain
2.7.3. Enveloper Sender Signature
2.7.4. Accept Bounces Only for Real Users

In this chapter, we look at various ways to weed out junk mail during the SMTP transaction from remote hosts. We will also try to anticipate some of the side effects from deploying these techniques.

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