pop problem

[1761951]

Hi

newbie just installed a mailserver; seemed to work alright but now discovered a pop problem:
- smtp sending mail from account xxx@mydomain.com to anybody@everydomain.com is no problem
- pop retrieving mail from account xxx@mydomain.com is no problem

but I can't send mail from account yyy@someotherdomain.com to xxx@mydomain.com. The mail just disappears without 'undelivered' messages.
DNS records are:

@ A ip.ip.ip.ip
@ MX 10 mydomain.com.

Anybody any clue/

Thanks
Rob