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    RFC2821 (Simple Mail Transfer Protocol) obsolete 821
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    RFC974 (MAIL ROUTING AND THE DOMAIN SYSTEM)
    RFC1123
    RFC1651  RFC1652  RFC1653  RFC1854  RFC1869  RFC2554 (SMTP Service Extension)
    RFC1468 (ISO2022-JP)
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    RFC2047 (MIME) obsolete  1522
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    RFC2683 IMAP4 Implementation Recommendations
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