So if you've got three MX's all at the same preference level the sender will randomly choose one and if it can't connect will pick another.
iow, all MX at a preference level will be tried before trying MXs at lower preferences.
If any one of the MXs gives an SMTP response, the MX algorithm is terminated.
ie, if "10" says SMTP "5xx" or "4xx" or "2xx", then the sending MTA should not try "20".
iow, another MX is a "try" fails only if no _SMTP_ response from an MX (tcp no_connect or refused doesn't count as an SMTP try).
Len
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