Hi Tim
How do you know?
Can you give us a reason why you think this is the case? Is you Sme server 5.1.2 patched up with the latest 5.1.2 patches? these will be quite old now as there will be no more patches this version of SME (I think i am right to say that?) I think for sme 5.1.2 Update 3 was the last? not sure. If you are Worried It might be an idea to upgrade to 5.6 with Update 5, or 6. or update to version 6 final release.
I think i am correct in saying it is considerd that 5.1.2 is now unsecure and might be ok for internal use but not as a server and gateway. I am sure there are far more people in this forum who know more on this than i do.
Best Regards
Adrian