Custom Domain with Subdomain(s) and Mailhop relay -

I am looking to do a custom domain to host my own email server and having a difficulty / confusing finding different options on setting up the accounts.

Maybe someone has some experience with this setup and can give me some ideas :-)

Questions I have are

1. If I register my domain with dyndns how many subdomains are supported and is there a charge for each

2. Can each subdomain point to different static or dynamic address. Examples maybe www, ftp may point to one server, home. may point to the dynamic

3. If my register is "hosted" somewhere else does this change how the subdomains are managed / supported

Thanks

chris