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どうも、skel.103Mです。ご返答どうもです。 メールアドレスと送信先ホストの件、了解しました。私なりに調べた ところ、RFC2821の「5. Address Resolution and Mail Handling」で、 > are generally discouraged. The lookup first attempts to locate an MX > record associated with the name. If a CNAME record is found instead, > the resulting name is processed as if it were the initial name. If > no MX records are found, but an A RR is found, the A RR is treated as > if it was associated with an implicit MX RR, with a preference of 0, > pointing to that host. If one or more MX RRs are found for a given > name, SMTP systems MUST NOT utilize any A RRs associated with that > name unless they are located using the MX RRs; the "implicit MX" rule > above applies only if there are no MX records present. If MX records > are present, but none of them are usable, this situation MUST be > reported as an error. とありますね。なるほど。ちなみに、RFC974にも似たような記述があり ましたね（現在はHISTORICですけど） ただ、分からないのは、なぜこのように定められているのか、その理 由が私にはよく分かりません。。私には直感に反するように思うんで すが…… |