DNS – Is an MX-Record Required on a Domain to Send Emails Only?

Consider registering with dnswl.org. There are also a few open services which will verify your configuration.

• Definitely get rDNS right, and use a subdomain for the host. eg. mail.example.com not example.com. Ensure you have a proper static IP address which is not listed with spamhaus.org. Avoid using any part of your IP address in the DNS name.
• Match the mailserver name configured in your software to the DNS name.
• Do setup the required postmaster and abuse email addresses.
• Ensure callouts for the sender addresss works, at least for the target server.
• Consider implementing Bounce Address Tag Verification (BATV) to allow verification of bounce notices.
• Definitely do SPF correctly. While spammers use it, it will help prevent spammers from using your address. Also do SPF for the server. See the FAQ at open http://www.openspf.org/.
• DKIM will authenticate the signed contents which may be useful for your purposes. It doesn't have much impact on spam scores. I am finding that a lot of the existing signers have broken implementations.
• Do setup appropriate policies for email being sent and received. Implement as many of these in the server as possible to limit incoming email.
• Limit the servers you relay for to those inside your LAN which need to send email. Only accept email for your domain.

I have a post on Running an Email Server with some other comments. Its a bit of a rant as several legitimate posters send email that looks very much like Spam due to poor email implemenations.

EDIT: Typically services like smtp, mail, and www have service related names (sub-domains). In some cases these will have numerical suffixes like smtp2 or www1. This makes it simple to provide the services on different servers simple. The servers usually have their own hostname (sub-domain) which is unrelated to the sevice(s) they provide. These days the domain may be used for web traffic, but is is often immediately redirected to the web sub-domain.

As Dom said, you don't need an MX record if it's an outbound-only mail server, the MX record is only needed for inbound mail. There is no such thing as an "A-record for an IP-address". Domains can have A-records, IP-addresses can only have PTR-records (Reverse-DNS). Most mail services only require you to have some PTR-record for your IP that does not look like a dynamic record (e.g. NOT 12-34-56-78.reverse.domain.com).

Instead, it should point to some domain name resolving to the server's IP. Some really strict mail providers will also check that your mail server's HELO corresponds with your PTR record, so it's a good idea that they're the same.