lammert - 8:40 pm on Nov 17, 2009 (gmt 0)

In your current setup, the MX record points to a CNAME which is not allowed according to RFC 2181. It must resolve directly to an A record.

There are three valid setups:

Option 1: (bulky, but correct)
mydomain.com A w.x.y.z
mail.mydomain.com A w.x.y.z
mydomain.com MX mail.mydomain.com 10

Option 2: (redundant use of domain name)
mydomain.com A w.x.y.z
mydomain.com MX mydomain.com 10

Option 3: (shortest version)
mydomain.com A w.x.y.z

Yes, the last option is also valid because the SMTP standard automatically uses the A record of a domain name if there is no MX record available. This is to be backward compatible with the oldest specifications. MX records are a "new" invention to separate the main domain IP and the mailserver accepting mail for that domain.

The PTR record should be valid, but there is no need to make it the same as your MX record.