Heh, we have two threads going for this issue. As I just mentioned in the other thread, DNS is a multi-step process. Yes, technically I could change the MX records for your domain to point to google's mailserver instead o here. However, as I have to manage records for dozens of domains, I've written automated tools that routinely overwrite those settings. E.g. whenever someone changes their account settings, it'll rewrite all DNS records just to make sure things are clean. I'd have to reprogram several of the management scripts to allow for external MX records... and, no offense, but I'm just not wanting to put that kind of effort in.
As I mentioned on the other thread, there's no reason you shouldn't be able to get this working using godaddy's DNS (the catch is, you have to pay them for DNS service, but it sounds like you already signed up for that.) If I understand correctly, all you should need to do is point the MX record(s) at whatever mailservers google wants, and then point the A records for the domain (and the www., which is sometimes a CNAME) to the IP of xepher.net. No end-user should know any difference in how it's setup. The one exception (as I mentioned) is if/when the XN server moves (or switches to a backup in an emergency) your domain wouldn't be updated automatically. You'd just have to get in touch with me and get the new IP/info to set though, so still not a huge deal.
If you have trouble with this, point more info about what exactly your trying to do. That is, the instructions/DNS info google wants you to set, and what you already have set at godaddy. We (I) can help you figure it out most likely.