DNS management is very important for the structural design of your own domain. Many internet service providers offer DNS services but most of them are very limited and allow only basic features. However, there are free services that offer full access to all DNS features. xname.org and dns.he.net are two of them. Caution! You have to know how DNS works. This is for experienced users only!
With dns.he.net you can easily setup a private mailserver even with a dynamic ip, through dyndns-client support for arbitrary A-records (means name->IP mapping). For this purpose you can use e.g. ddclient which is included in the repositories of all major linux distributions.
All you have to do is to setup “external DNS management” with your domain hoster. It takes some time to sync all DNS-servers throughout the world so be patient while waiting for it to work after the initial setup (it may take one or two whole days). It’s worth it!