Outbound spam

Hi,

I have issue when our company user is trying to send emails to partner domain and gets back always same message from MessageLabs:

#553 Message filtered - Outbound Spam (#5.7.1)

Partner domain is already added to approved senders under Anti-Spam in messagelabs.

Just wondering maybe you have some suggestions what could be reason for this as i cant see anything else wrong in Messagelabs settings

Thanks.