Hi,
Sorry for the late reply. I can confirm that the reputation of that IP is good and the IP 178.89.247.36 is not on any internal blacklist. I can also confirm that you should not be trying to connect to cluster3a.eu.messagelabs.com, this MX record will always be the second MX record and should not be used for sending emails. Emails should always connect to the Primary MX record cluster3.eu.messagelabs.com. If a customer of ours is using that cluster their MX configuration should always be as follwed:
10 cluster3.eu.messagelabs.com
20 cluster3a.eu.messagelabs.com
All emails will route through the primary cluster and the second MX is never used.
Please do let me know if you need any further information.
Kind regards
Kevin Brosnan