Well if you are using ePO 5.9 and you want to manage MSME 8.5 then you would need compatible extension.
The Extension for MSME 8.5.1 in software manager is compatible with MSME 8.5.1 and its 3 available HFs.
I don't believe the MSME 8.5 original extension will even install in ePO 5.9 - don't think its on the compatibility list.
If you manage 8.5 with latest extension then there are incompatibilites in internal components and items such as antispam B&W lists may not work as expected in the actual scan process.
You could remain with with unmanaged MSME at current level but most if not all the issues you could face with that version would have been resolved in MSME 8.5.1 or possibly any of its 3 Hfs.
Not sure also if MS are providing support for that version of Exchange - End Of Exchange 2010 SP2 Support – 250 Hello
i'm actually in holliday so far from my server but from memory, for MSME in ePO 5.9, you need 8.51 version with latest extension AND latest hotfix. I'll check asap this information
I uninstalled the whole thing, because the newest version of ePO seemed to have trouble supporting their own previous products like VSE etc which I couldn't risk to continue with.
Ended up installing ePO 5.3 through ePA Suite installer, upgraded my MS Exchange to 2010 SP3 and now im in the process of configuring it with MSME.
Thanks for all the help