MX Records.

I am hoping somebody can help me out.  I am not a DNS expert and would like some input about the following scenario.

We host our own exchange server and I am in the process of testing routing our email through a 3rd party spam filtering company.

To do this I was required to create 4 mx records for my domain directing the mail to the 3rd party servers where it is filtered and then forwarded to my in-house exchange server.

The 4 mx server records  all have a different priority.

My dilemma.  The 3rd party company requires that all email addresses be listed on their servers.  This in itself is not a problem unless for arguments sake I create a new user on our server and forget to duplicate it on the 3rd party server.  Obviously, email sent to that address will fail at the 3rd party server.

My question is,  If I created a fifth MX record pointing directly to my server and put it last in the priority list, "Will the email get through"  Am i right in thinking that once the first 4 have failed (Due to the address not being listed on the 3rd party servers) that the last server, ie -Mine will be tried with success.  If so I understand that the emails won't be filtered for spam but at least I won't be losing emails.

Also would this affect the delivery to the 3rd party servers for valid email?
LVL 14
ldunscombeAsked:
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smckeown777Connect With a Mentor Commented:
Not exactly...adding a fifth MX record is fine...but it will only ever get traffic if the first 4 are offline/not available...nothing to do with the fact that an email address doesn't exist...MX records are related to the service itself...so if any of the first 4 records are alive then the new fifth record will never get hit...
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ldunscombeAuthor Commented:
I was afraid of that.

Any suggestions you could offer as to how I might be able to achieve my goal would be greatly appreciated.
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Esteban BlancoPresidentCommented:
I would think that the MX records would just need to be pointed to the third party vendor and that's it.  In an scenario like setting up Postini for example, the whole point is for all email to go through their servers without touching yours and delivering only the good email.

So if you were to have Go Daddy as your domain provider, you would create the MX Records in Go Daddy's DNS manager and point them to the third vendor spam filter (whomever that is) so the email routes there first.  I would go with whatever priorities they gave you and whatever TTL is recommended.

The reason they want 4 created is for redundancy so the spam filter has architecture flexibility.

You can create MX records in your server as a backup but use the lowest priority possible.  Your email most likely will never use that MX record.

Let me know if all this makes sense.  :)

-Esteban
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smckeown777Commented:
Realistically there is no other way...since all email traffic is hitting the 3rd party first then in reality you will always need a mailbox on their end if it exists on your server...I don't think getting around this is possible...

The other way to do this is install a spam filtering product on your server(like GFI MailEssentials for example) which means no need for 3rd party...others may have other inputs on this...
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ldunscombeAuthor Commented:
The main reason why I would like to be able to do this is because I have numerous mailbox's for ex-employees.  While they are not active as such anymore I would still like to receive emails sent to those addresses without the need to duplicate them with the 3rd party.

But if it can't be done,  It can't be done.

Thank you for your help.
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