Cannot send or receive email ('Message not sent. Server replied:)

[Numicus]

Hi all,
I cannot send or receive any e-mail

- The e-mails do not work, when using webmail I do not receive messages and when sending I get the error: 'Message not sent. Server replied:' with nothing behind it.
- When I add the POP3 account to a program like outlook, it also won't receive messages
- When telnetting the POP3 server, loging in etc works great, messages aren't received though..
- The domain works fine on the server, dns-settings are on external dns and work fine

Do I need to post any logfiles?