Mail and DNS

Contents

MX records

To run a 'public' mail service you need to have a DNS MX record. This is used by other mail servers to decide what the hostname is for the server that looks after email for your domain.

The references standards that DNS servers conform to are at …

• ftp://ftp.rfc-editor.org/in-notes/rfc1034.txt
• ftp://ftp.rfc-editor.org/in-notes/rfc1035.txt

<yourdomain.com> MX 0 <mailserver domain name>

Where the number is the priority assigned to that mail server. If the lowest numbered server is not available, then sending should default to the next if it exists.

MX and CNAME records

When a general query is sent to a DNS server, most will reply with a CNAME record first and *might* try to resolve that to an IP address before referring to an A record.

So if you try to use a MX to a CNAME record, you may see some very odd results. Basically its a bad idea, and here's why...

Most mail servers will not check that the MX record they receive resolves all the way to an A record. Instead they will just take the domain name attached to the MX record for granted and apply that to headers for sent emails. Which will mean reverse lookup checks will fail unexpectedly, and your emails may either bounce or be delivered into the void.

The solution to this is to make sure that mail server domain names point to an A record.