From a compatibility standpoint, the versions of the emailgw probe you could probably find (2.73 through 2.84 at least) will work with UIM 8.5. There are some behavioral idiosyncrasies of some of the versions - like one of them truncates the subject line of the message sent to 240ish characters - makes it more SMTP compliant but if you were using one of the older versions that didn't truncate and were intentionally sending long subjects, the newer probe version might be an unpleasant surprise.
But otherwise they all "work".
On the other hand the documentation isn't especially good for the probe and some of the error messages you see in the logs sound bad but are "expected". This particular probe intentionally does specific things when there's missing data - like overloading the "To:" field to identify either a user account or a profile. There you'll get the "account not found" error message but then it goes ahead and uses the same named profile and all's good. Makes troubleshooting hard and you have to be careful about name collisions.
-Garin