4. The check_host() Function ......................................12
   4.1. Arguments .................................................12
   4.2. Results ...................................................13
   4.3. Initial Processing ........................................13
   4.4. Record Lookup .............................................13
   4.5. Selecting Records .........................................13
   4.6. Record Evaluation .........................................14
        4.6.1. Term Evaluation ....................................14
        4.6.2. Mechanisms .........................................15
        4.6.3. Modifiers ..........................................15
   4.7. Default Result ............................................16
   4.8. Domain Specification ......................................16
5. Mechanism Definitions ..........................................16
   5.1. "all" .....................................................17
   5.2. "include" .................................................18
   5.3. "a" .......................................................19
   5.4. "mx" ......................................................20
   5.5. "ptr" .....................................................20
   5.6. "ip4" and "ip6" ...........................................21
   5.7. "exists" ..................................................22
6. Modifier Definitions ...........................................22
   6.1. redirect: Redirected Query ................................23
   6.2. exp: Explanation ..........................................23
7. The Received-SPF Header Field ..................................25
8. Macros .........................................................27
   8.1. Macro Definitions .........................................27
   8.2. Expansion Examples ........................................30
9. Implications ...................................................31
   9.1. Sending Domains ...........................................31
   9.2. Mailing Lists .............................................32
   9.3. Forwarding Services and Aliases ...........................32
   9.4. Mail Services .............................................34
   9.5. MTA Relays ................................................34
10. Security Considerations .......................................35
   10.1. Processing Limits ........................................35
   10.2. SPF-Authorized E-Mail May Contain Other False
         Identities ...............................................37
   10.3. Spoofed DNS and IP Data ..................................37
   10.4. Cross-User Forgery .......................................37
   10.5. Untrusted Information Sources ............................38
   10.6. Privacy Exposure .........................................38
11. Contributors and Acknowledgements .............................38
12. IANA Considerations ...........................................39
   12.1. The SPF DNS Record Type ..................................39
   12.2. The Received-SPF Mail Header Field .......................39
13. References ....................................................39
   13.1. Normative References .....................................39
   13.2. Informative References ...................................40

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