Hello!
Third-parties routing mail on your behalf should not be attempting to route mail out through our systems. It would not be an inbound or outbound route with us. The third-party server should use DNS to send mail to your MX Records the same as any party sending mail to you. If they are unable to route via MX and want a specific server, then you would provide cluster1.eu.messagelabs.com. The fact they are using your own domain doesn't mean they should be added as routes, it is no different than other mail coming in from any other sender.
Note: You would need to make sure they are properly authorized in your SPF Record if you have one in DNS. They should be able to advise you what to add as the SPF Record.
For clairity:
Outbound Routes are servers under your care and control, routing mail only for your organization, that are allowed to send outbound through us to reach a third-paty recipient. This would generally be your mail server.
Inbound Routes are where we deliver mail for a domain once it enters our system. This would generally be your mail server.