Just to follow up for those interested: I went with Smartermail 4.1, and it's working fine so far. It integrates with PLESK 7.6.1 on Windows (as with all things PLESK it's a bit of hammering a square peg into a round hole, but it does -- eventually -- work). Beware, however, that Smartermail on Windows is a memory hog, and perhaps not particularly well suited to a VPS scenario (particularly not in my case with CF and PLESK both running on the box in addition to the mail server).
Configuring spamassassin is a bit frustrating: if you leave the settings "as is" with PLESK running the default included spamassassin and Smartermail trying to do the same thing, things go really badly really quick. PLESK will spawn countless smmailfilter.exe processes and bring your server to its knees by maxing out processes, while Smartermail will launch one monster spamd.exe process that will slowly grow to max out available memory. The trick seems to be to go in and turn off PLESK spamassassin (of course, true to the PLESK nature, there's nowhere logical to do that -- it seems to be a matter of simply checking off the "use serverwide settings" box in the spam management). But overall, the tech guys at Smartermail recommend using only the greymail filters in a VPS/limited resource scenario -- much less resource intensive, and so far it looks to be pretty efficient.
So, FWIW: Smartermail is a viable option as an alternative to mailenable; it has a really nice set of features and very decent support, incl. an active community. But YMMV.
