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Public MX Records and Priority?

Posted on 2013-05-27
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Last Modified: 2013-06-19
Can you explain a public MX record and oriority?  If an MX record is attached to a public IP and friendly name how does the priority work?  Is a priority 5 higher than a priority 10?
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Question by:iNetSystem
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by:Stelian Stan
ID: 39199524
The lower the number is the higher the priority. Like you said 5 is higher then 10.
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iammorrison earned 2000 total points
ID: 39199528
A Mail Exchange (MX) record is a DNS object that is used to resolve email domains for SMTP. Even if DNS records exist for domain yourcompany.com ( if they have a website for example) if this domain wants to receive email, then it must also have a valid MX record pointing mail traffic to authorized mail server(s). Priority is given to the MX record with the LOWEST numbers first. To use your example, a record with a priority of 5 takes precedent over a record with priority 10.
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by:EddyvanOpdorp
ID: 39199556
Correct, how lower the number, how higher the priority.
If you have 2 record with the same priority it is random. This is called round robin.
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by:Amit
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by:Simon Butler (Sembee)
ID: 39199971
Of course everything about priority is theoritical.
In practise you will have email go to all hosts in the MX record, and if you have one with a higher cost then it will get more spam, as spammers target them.

I work on the basis that the priority means nothing, I don't use it with any of my clients. If they have multiple servers able to receive email then they all get the same cost - 10.
I don't believe that there is any round robin in MX records.

Simon.
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by:Craig Beck
ID: 39202633
In my experience the priority is not theoretical at all.

I have a MX record pointing to my primary SMTP server which is authoritative for my domain, but I also have a MX record pointing at a fallback SMTP server which isn't authoritative for my domain - it just acts as a relay so that if my SMTP server becomes unavailable it will store-and-forward.

Some of my clients have geographically distributed mail servers which manage the same domain.  The priority is the same on all MX records and mail delivery is totally random (as implied by round-robin DNS resolution).  If the SMTP server with the lowest priority is unavailable, the next SMTP server with the same priority isn't used though - the one with the next preferred priority is used.
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