MessageLabs don’t have any block lists of their own, which is why they don’t have a means to request being removed from it. They use other lists, such as SBL etc. Nor do they maintain lists of blocked email addresses or domains. Their individual customers can choose to block-list your domain or email address, so it’s possible that is what is happening here – but unlikely?

If the people you are trying to send to want to receive your mail, then clearly the best course of action is for them to contact MessageLabs as their customer and complain that your email to them is being blocked. If you are able to provide details of who the message was sent to, when, and what the subject was, MessageLabs should be able to identify very quickly what happened to it (as indeed should the recipient themselves). If the recipient is unwilling to do that then it suggests that they don’t want your mail, in which case that’s probably why it’s being blocked!