The problem is less something specific to Mercury than a widespread problem in the way SSL is implemented generally. There are many different libraries and versions of libraries for doing SSL and not all of them interpret the specifications in the same way.
Now, with MercuryP, MercuryI, MercuryC and MercuryD, it's not too much of an issue, because they're only talking to a fairly limited range of implementations, so if there's a problem, it will become obvious at an early stage. Enabling SSL in MercuryS, though, exposes you to every implementation in Christendom, with all their vagaries and differences. For this reason, I generally do not recommend enabling SSL in the SMTP server - and will not be recommending it until the industry settles down considerably in this area.
-- David --