well, I am assuming that you are leaving your DNS management to DynDNS DNS servers.
I am also assuming that your server is up and reacheable from the Internet.
let's say that your linux box is called linux.mydomain.com and you can see it from outside (i.e. ssh linux.mydomain.com)
In this case, (I am not familiar with DynDNS interface) you should have an applet to configure Mail settings in the DynDNS portal.
a value that would work is exactly the one above
Linux.mydomain.com and with MX value to 10.
Note:
If you cannot reach your server from the internet at all then you may still be experiencing problems with passing your dynamic Ip address to DynDNS.
to verify this ping your domain, write down the numerical IP you will get from your ping probes and the visit
the above page will return your public IP address
Make sure that the two are the same.
QatQat
Life is what happens when you are making other plans.