MX Records and RDNS lookups

Posted on 2011-10-04
Last Modified: 2012-06-27
I am moving my company email over to MS Online. Most of the process is already done except moving my MX records to MS Online. Right now my email comes into my exchange server and then forwards to MS Online. Once I change the MX records, I want to make sure that I can still use my current mail server for SMTP relay without the outgoing mail failing RDNS lookups. Does anyone know what DNS entries I need to make so that both mail servers can send out email, but I only want MS Online to receive mail.

Question by:baselhamad
    LVL 6

    Expert Comment

    Just change your domain MX to MS Online Mail servers.
    Your server will always be allowed to send mail and resolve recipients MX by itself, as long as it respects RFC 822 security and uses a public dns server (or there must be a forwarder on your internal DNS servers so your mail server can resolve external MX)
    Keep a valid reverse DNS and FQDN to your public IP in your company DNS Zone , there will be no problem to relay mails using your server without jumping on MSOnline ones.


    LVL 21

    Expert Comment

    Aside from MX records and what Gerald said, you should create an SPF record for your domain and put the sending IP's of your server(s) and MS Online server(s).

    Author Comment

    I am not sure of the syntex of setting up an SPF record. Can anyone point me in the right direction?
    LVL 21

    Accepted Solution

    You add a TXT record to which lists the IP's that are allowed to send mail for

    Here is a very simple example:
    "v=spf1 ip4: ~all"

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    "v=spf1 ip4: ip4: ~all"

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    There is a lot of info out there on SPF record syntax, however there are best practices that should be followed, and not a lot of people understand those well.  I, on the other hand, understand it all very well :)  Things like using 'a' 'mx' and 'ptr' mechanisms when they are not needed, and overall syntax issues are common.

    First I would contact MS Online to get their sending IP's, you can probably find it in a FAQ on their site.  Then get your IP, do some research on SPF records, and then paste your record here and I will review it and give any pointers, sound good?

    LVL 27

    Expert Comment

    if you dont already have an SPF record you wont be affected by changing the MX record to the MS online address.
    Keep your rDNS entry in place and consider checking if you can add one to the MSoffice address you have been given, as their system will send mail and may need the rDNS entry for it ot be accepted.

    if you have an SPF record, ensure that both the MSoffice server and your own server are listed.

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