afaik the protocol wont deliver to 2 mailservers at the same time - if you set 2 mx records to have the same weighting it'll probably roundrobin (thats dns' usual behaviour). afaik if you want mail to go to 2 hosts, you'll have to forward it yourself...
Some mailing software allow you to relay the incoming messages to another server after inital processing. You could do this replicate to replicate the 2 desitnation servers.
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