Hi Steven
I checked the IP you gave and do not see any issues with it, so as you say it may not be the IP that is causing the issue.
I see you say that the mail is being sent outbound. Can I ask from which domain this is being sent, and also which smarthost are you pointing to?
It may be best to directly message me this information so it is not then public.
Thank you
Ian Tiller
Tier 2 Senior Technical Support Engineer