Multiple MX Records

why is it that when performing a query with dig (linux) for a domain's MX records do some of them come back with multiple MX records and each point to different IP's? The only thing I can think of is for disaster recovery and back-up. Is there something other meaning to this? I typically see this with companies that have their email hosted by third party.  Thanks in advance.
J-VasAsked:
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BWaringCommented:
More than one MX record means more places to deliver mail. Start at higher priority (lower number) and work from there. If two priorities are the same, then alternate (gateway dependent). That's it. Those rules can work themselves in to many 3rd party/non-3rd party scenarios regarding primary delivery, overflow delivery, redundancy, capacity planning, etc...

If you have a specific example or issue you are concerned about, let us know...

For example, you could have more than one mail gateway at different edges of you WAN, accepting mail for your domain. Each of these could have the same priority. If either is down, the other will be used.... one example... there are many more....
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Glen KnightCommented:
If it's a hosting provider they will have multiple connections & IP's for redundancy, therefore will require an MX for each SMTP entry point.
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saigasmithCommented:
It also allows for priority as each MX record has a priority (lower the number, higher the priority). The MX record with the highest priority is the actual target computer where mail boxes are located. The other MX records designate the backup computers in case the actual target computer is not accissible. They store the e-mails temporarily, and periodically attempt to redirect e-mails to the actual target computer.

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Glen KnightCommented:
Sorry, but that's not strictly true.

The priority does not necessarily mean the lowest number is where the mailboxes are stored.

They could be stored at both, or both routes could go to the same place just for different ISP's/IP addresses as used for redundancy.
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BWaringCommented:
Some additional notes on priority... priority just indicates the order that the various mail servers should be contacted;  the lower number is a higher priority. So if three mail servers have priorities of 10, 20 and 30, try the 10 server first, if that fails, try the 20 server, and if that fails try the 30 server... what constitutes failure and when to try the next higher preference server is not necessarily the same in all SMTP gateway implementations....

Anyway, more info on MX records can be found here: http://en.wikipedia.org/wiki/MX_record
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J-VasAuthor Commented:
Just to make sure, there is no other meaning to multiple MX records for a domain? Even if not using a  third party hosting company?
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