I hope I can explain this.....
Here is the details of my machines hostname settings
Hostname: mail 
Domain: thebigsmell.com 
Location:  SelfLocalRemote  
Local IP:  no entry
Ethernet address:  no entry 
Publish globally?  
Global IP:  box is ticked
Now on the dyndns.org site I have no mx records as I just have the one box hosting the www as well as mail ftp etc etc
I can go to my domain 
www.thebigsmell.com/webmail and send mail to anyone, people can send me mail to the thebigsmell.com account without it being bounced or anything like that, but when I check my inbox using the webmail there is still no mail. This is fine if I am only interested in sending mail but useless if I want to be able to read my mail 

Oh forgot to mention the thebigsmell.com is also a virtual domain
Thanks in advance for your help