Blackberry users do not receive email from inside the domain

[supamaki]

I have 3 blackberry users who can receive emails when I send them from an email address outside our domain but when I send them from inside the domain they don't get them.
We're using POP3, and if I log into the webmail client, I can see the emails, they just aren't getting to the devices.
Can anyone shed any light on this??

Thanks

[supamaki]

As a follow up, there is an exchange server, and these users have accounts in Active Directory with exchange mailboxes. Exchange is grabbing emails from inside the domain and trying to send them to their exchange mailboxes, when I don't want them to, I just want them to use POP3