Hello,
If you have your own domain with registrar services and you want to use you mail domain using google services like google apps http://www.google.com/a you only need to set up fine your DNS setup and to read some steps detailed by google.
Here is and example using tinydns to use google apps:
# yourdom = Your Domain :)
.yourdom.com:ip:ns1.dom.com:259200
.yourdom.com:ip2:ns2.dom.com:259200
# to google APS mx
Cgoogle6879112e21eab44.yourdom.com:google.com:3600
@yourdom.com::aspmx.l.google.com.:1:3600
@yourdom.com::alt1.aspmx.l.google.com.:5:3600
@yourdom.com::alt2.aspmx.l.google.com.:5:3600
@yourdom.com::aspmx2.googlemail.com.:10:3600
@yourdom.com::aspmx3.googlemail.com.:10:3600
@yourdom.com::aspmx4.googlemail.com.:10:3600
@yourdom.com::aspmx5.googlemail.com.:10:3600
‘yourdom.com:v=spf1 a mx include72google.com,include72googlemail.com ~all:3600
Now, you can enjoy using google with you domain!
See you.