Ecco Pro had a cult following. Unfortunately the software was sold a couple times and ended up at Net Manage, who killed it (along with a few others ).
But you can download an old version for free, which runs on Win32 systems. Here's one article about it.
converting to MsOutlook http://support.microsoft.com/?kbid=164046
Sept'2007: Scott Rosenberg notes someone building around it again, with Lua.
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