I haven't used dyndns so this might be a bit vague. If anyone else has they might be able to offer better advice.
You need to tell dyndns what your IP address is. If you're on a connection that changes IP periodically then you'll need a way to update your dyndns record. That's a separate problem and probably best to try they're help files or forums.
Once you've entered your IP address into your account all requests to your domain (example.mine.nu) will be sent to your modem/router.
If you're using a router than you need to set up some kind of port forwarding from the router to the machine that runs the service you're trying to serve. Obviously forward the required port. 25 for mail, 22 for ssh, 80 for web. If you have access to an account on an outside machine you can now test that with your new found telnet skills
By using telnet (from the outside) you should get a connection to the appropriate service.
MX records are simple a subset of a DNS record used specifically by email. It means that you can host your email on a separate machine to your webserver. In your case you simple want your MX record to point to the same IP as the rest of your domain.
In summary what you want is;
1) Correct IP address in dyndns
2) Same IP for your MX record
3) Set up port forwarding on your router to pass the connection to the correct machine.
4) Set up a service on the receiving machine.
Sorry that's not more specific. You best bet from here is to google around for a few examples on seting up web/mail servers on your distribution.
One thing to always keep in mind. If you open up a service to the outside world it's very important that you keep that machine up to date and consider any possible insecure setups. It's a hostile world out there on the net.
Good luck, don't give up and remember google/man pages are your friend.