messagelabs.com do not deliver our messages to our clients

We are sending messages to a messagelabs managed domain (relay=cluster3.eu.messagelabs.com).

Our IP (195.201.21.207) is marked as "The IP address you submitted, 195.201.21.207, does not have a negative reputation and therefore cannot be submitted for investigation." at the IP investigation query.

We do not receive any kind of blocking message, they look as delivered at our logs, and do not appear at the clients inbox (nor spam folder).

What can I do to have my emails delivered to my customers?

Regards,

David.