MX question

[Barry Margolin]

In article <a8t8dq$pt2 at pub3.rc.vix.com>,
William Stacey <staceyw at mvps.org> wrote:
>What the advantage to doing this:
>;; ANSWER SECTION:
>microsoft.com.          6789    IN      MX      10 maila.microsoft.com.
>microsoft.com.          6789    IN      MX      10 mailb.microsoft.com.
>microsoft.com.          6789    IN      MX      10 mailc.microsoft.com.
>
>;; ADDITIONAL SECTION:
>maila.microsoft.com.    6789    IN      A       131.107.3.125
>maila.microsoft.com.    6789    IN      A       131.107.3.124
>mailb.microsoft.com.    6789    IN      A       131.107.3.122
>mailb.microsoft.com.    6789    IN      A       131.107.3.123
>mailc.microsoft.com.    6789    IN      A       131.107.3.121
>mailc.microsoft.com.    6789    IN      A       131.107.3.126
>
>As opposed to just listing more MX's like:
>;; ANSWER SECTION:
>microsoft.com.          6789    IN      MX      10 maila.microsoft.com.
>microsoft.com.          6789    IN      MX      10 mailb.microsoft.com.
>microsoft.com.          6789    IN      MX      10 mailc.microsoft.com.
>microsoft.com.          6789    IN      MX      10 maild.microsoft.com.
>microsoft.com.          6789    IN      MX      10 maile.microsoft.com.
>microsoft.com.          6789    IN      MX      10 mailf.microsoft.com.
>
>;; ADDITIONAL SECTION:
>maila.microsoft.com.    6789    IN      A       131.107.3.125
>mailb.microsoft.com.    6789    IN      A       131.107.3.124
>mailc.microsoft.com.    6789    IN      A       131.107.3.122
>maild.microsoft.com.    6789    IN      A       131.107.3.123
>maile.microsoft.com.    6789    IN      A       131.107.3.121
>mailf.microsoft.com.    6789    IN      A       131.107.3.126
>
>Is the advantage round-robin?

Some mailers have a limit on the number of equal-preference MX's that
they'll try, possibly only one.  So if a server only tries one MX record,
it will still see both addresses corresponding to that server, and may try
both addresses.  If you had six MX records, only one address would be tried
in this case.

-- 
Barry Margolin, barmar at genuity.net
Genuity, Woburn, MA
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