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KBA80004: Querying a Domain for Mail (MX) Records

Knowledge Base Article #: 80004
Applicable Versions: N-central 6.x, 7.x, 8.x, 9.x
Date Created/Updated: April 27, 2011

Introduction

This article will show you how to query for a domain's mail server. Sometimes this can be handy when attempting to resolve mail issues. We'll use n-able.com as an example:

  1. Click on Start, point to Run and type: cmd.exe and press <Enter>.
  2. Type: nslookup and press <Enter>.
  3. Type: set type=mx and press <Enter>.
  4. Type: n-able.com and press <Enter>.
  5. Here is what the results look like:
    [root ~]# nslookup
    > set type=mx
    > n-able.com
    Server:         216.138.0.4
    Address:        216.138.0.4#53
    
    Non-authoritative answer:
    n-able.com      mail exchanger = 200 n-able.com.s8a2.psmtp.com.
    n-able.com      mail exchanger = 300 n-able.com.s8b1.psmtp.com.
    n-able.com      mail exchanger = 400 n-able.com.s8b2.psmtp.com.
    n-able.com      mail exchanger = 100 n-able.com.s8a1.psmtp.com.
    
    Authoritative answers can be found from:
    n-able.com      nameserver = dns.n-able.com.
    n-able.com      nameserver = dns2.n-able.com.

Now, let's take a look at the results:

n-able.com      mail exchanger = 200 n-able.com.s8a2.psmtp.com.
n-able.com      mail exchanger = 300 n-able.com.s8b1.psmtp.com.
n-able.com      mail exchanger = 400 n-able.com.s8b2.psmtp.com.
n-able.com      mail exchanger = 100 n-able.com.s8a1.psmtp.com.

Each entry represents a mail server that is configured to accept e-mail for the n-able.com.com domain. Note that each entry has a unique FQDN at the end (i.e. n-able.com.s8b1.psmtp.com), and that each FQDN is lead by a number (i.e. 2,4,6,8). Those numbers tell you which server to send e-mail to first - they indicate the priority of the server.

Note: the lower the number, the higher the priority (i.e. you'd send the e-mail to the FQDN with a priority of 2 before you'd send it to the server with a priority of 4).

There are two common problems that can occur:
  1. Two unique FQDN's have the same priority:
    n-able.com      mail exchanger = 300 n-able.com.s8a2.psmtp.com.
    n-able.com      mail exchanger = 300 n-able.com.s8b1.psmtp.com.
  2. The priority given to the FQDN's are not consistent.
    n-able.com      mail exchanger = 1 n-able.com.s8a2.psmtp.com.
    n-able.com      mail exchanger = 3 n-able.com.s8b1.psmtp.com.
    n-able.com      mail exchanger = 400 n-able.com.s8b2.psmtp.com.
    n-able.com      mail exchanger = 45 n-able.com.s8a1.psmtp.com.

Whatever priority is going to be given to the MX records, they have to be consistent (i.e. 2,4,6,8 or 100,200,300,400 or 10,40,50,60). If they're not, it can cause all sorts of mail-type badness to occur.