My current setup is as follows (mail server runs on the same IP as my web server):
mydomain.com A w.x.y.z mail.mydomain.com CNAME mydomain.com mydomain.com MX mail.mydomain.com 10
I've also a PTR record from my ISP which currently points to "mail.mydomain.com".
The situation is that several major mail servers require a PTR record to be present in order to accept mail from my mail server. My question is, is it enough for the PTR record to point to mydomain.com instead of mail.mydomain.com ? Or should I just remove the "mail.mydomain.com" record and point the MX to mydomain.com, since there is really no point in having it other making my domain look cooler :P
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.