Migrating to new ISP

Posted on 2012-08-29
Last Modified: 2012-09-06
Hey all,

For years we have been running off a T1 and recently cable and it's much increased bandwidth have made it into the area. Our line was installed and now I need to migrate over to the new line. I have pretty much done most of the work by enabling load balancing on the server and tested that the throughput is working as it should with the new connection. I have matching rules for both WAN inbound and that is all working. However email does not accept or deliver when load balancing switches to the new circuit.

My research tells me I likely have to add an MX record for the server for the new IP, but I am only using and I dont have multiple A records published for this host. So do I add with the new IP, add that as the new IP and thats it? And what good does do me if nobody knows who that is. Do I set server2 in my local DNS table so it can translate to server.

A checklist if items would be sweet but I may not be using the right keywords when I search. Give me some terms for what I am trying to do here. I don't want to load balance 2 exchange servers, I basically want to load balance an exchange server between 2 ISPs, with the intent on dropping the original IP all together in a few weeks.


PS, I actually have 2 mail servers. One server collects mail for 2 domains and another, 1 domain. So I would be doing this for 2 boxes eventually. Also server1 is SBS2003 with exchange 2003 and the other is Server 2008 R2 with Exchange 2010 if that matters.
Question by:jmiller2781
    LVL 37

    Assisted Solution

    by:Jamie McKillop

    What you need to do is create a second A record called that maps to the IP on the other ISP. Now create a new MX record for your domain that maps to the A record. Make sure you have a PTR record for both IPs.

    I'm assuming you have configured your firewall on the second ISP line to allow port 25 to/from your Exchange server.

    LVL 1

    Assisted Solution

    I'm not sure if I got you right here but I'll try to explain our setup....
    We've got one leased line and one ADSL line (mainly used as backup).

    Our main mail server URL is, our secondary URL is
    The uses an IP address from the address range that came with the leased line, is using an address from the address range of the ADSL line.

    Both addresses have got an A record,
    141.x.x.x -
    81.x.x.x -

    MX records for both IP addresses - for us with different priorities (5 and 10) as we only want the second line as a backup line and not for load balancing.

    Internally, NAT is obviously set up to translate both public IPs to the internal server IP - plus reverse to translate the internal to the external when sending email.

    So, in short - if I don't misinterpret what you are trying to do - it should work this way

    - use an IP address from the new IP address range you have been assigned (if you have got one?) and publish an A record for this (e.g. sever2...)
    - Create a second MX record for the address (with the same priority as your main MX record if you want to load balance)
    - set up all internal NAT etc. that needs to be set up

    I hope this makes sense.
    LVL 14

    Accepted Solution

    Add two MX records for your domain, one pointing to your new (main) address, the other one to your old one.


    "A record" for "old" : (=new exchange server IP)
    "A record" for "new" : (=old exchange server IP)


    "MX record: priority 10 (gets mail by default)
    "MX record: priority 20 (gets mail if is unreachable)

    Author Comment

    I think you all get what I am saying. I will be tweaking this over the weekend. I will be sure to reply with any issues I had and close out the question.

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