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Introduction to MX Record and Email Issues
Below is an introduction to common MX record and email exchange issues.

What is an MX record or mail exchanger
An MX record is the record for a zone (domain) that specifies what machine serves as host for that domain's e-mail.

In other words, the MX records tell the world where to deliver the e-mail for your domains. Mx records should point to the fully qualified domain name (FQDN) of the server that handles your incoming email.

Incoming email servers are also known as SMTP servers (SMTP is simple mail transport protocol -- the language Internet mail servers use to communicate with each other).

By default MX records are set up to take advantage of the advanced email system built into each account. If you have a separate mail host (such as mail.yourdomain.com) you need to contact your service manager and request this change. There is normally a nominal fee for this installation.

Your zone/domain can have more than one MX to ensure that if one mail exchanger is down, your email will not be lost. This works by specifying multiple MX records for the same A (host/zone) record, with each different MX given a different preference ranking.

If the MX with the best (lowest number) preference cannot be reached, the sending SMTP server exhausts the list of available MX's from best (lowest number) preference to worst (highest number).

Common Mistakes or potential set up problems:

  1. Setting an MX record for a host by simply pointing the MX directly at an IP address.
    This is "illegal" and more than likely will cause that particular MX not to work. E-Mail and the MX must be directed to specific host name in FQDN format (i.e. mail.yourdomain.com not just mail) that in turn can be resolved to an IP address.
  2. Not ping'ing the IP address or fully qualified domain to verify it's setup before requesting an MX pointing.
    If you can not ping the IP address or FQDN email "will" fail (since the MX server is/has not been properly set up yet).


My E-Mail isn't working & I think DNS is the blame - how can I test it?
If you've already tried all the different configuration nuances in your SMTP server software, and are still receiving e-mail bounces, DNS may be to blame.

To test, you would run 'nslookup' and ask it what the MX record is set to for a particular domain.

Here's an example:

Enter:
$ nslookup - nsb

result will be:
Default Server:  b081.thevirtualcompany.com
Address:  205.149.189.81

Enter:
> set type=mx
> tvcorporation.com

result will be:
Server:  b081.thevirtualcompany.com
Address:  205.149.189.81

tvcorporation.com       preference = 0, mail exchanger = vs00.tvsecure.com
tvcorporation.com       nameserver = nsb.thevirtualcompany.com
tvcorporation.com       nameserver = nst.thevirtualcompany.com
vs00.tvsecure.com       internet address = 205.149.189.15
nsb.thevirtualcompany.com       internet address = 205.149.189.81
nst.thevirtualcompany.com       internet address = 216.112.11.178

If the mail exchanger is listed as an IP address (and not a domain like vs00.tvsecure.com) then you've found a case of where the MX is pointing may not work, see above.

The only other problems/discrepancies you might find is where the MX is pointing at an old or perhaps even a non-existent host.

If the MX is pointing at the correct hostname and that hostname points to the right IP address, then it's time to double check your mail server's configuration. One common mistake is to not add your new virtual domain to an 'allowed delivery' file. This file tells your SMTP server software which domains it should accept e-mail for.

Other common problems are missing or non-existent end point e-mail boxes, or even perhaps connectivity problems.

How do I backup my e-mail servers if there's a network outage?
As long as the DNS servers for your domain are up (or more specifically your MX records can be resolved to a valid IP address), most other SMTP servers on the net will store in cache any outgoing messages it needs to deliver to your downed SMTP server for a preset amount of time. After a certain interval of time, the SMTP server will send a copy of the e-mail back to the original sender, notifying them that the e-mail seems to be undeliverable, but that it will keep trying for X number of hours.

As long as an SMTP server can find an address for the endpoint mail exchanger, it'll hold e-mail destined for that endpoint until the endpoint is reachable. As long as other folks out there on the net can lookup your MX record, you probably won't loose any e-mail. So as long as at least one of our DNS servers is reachable (and assuming we provide your name service), a network outage should have little effect on your e-mail traffic.

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