An oldie but a goodie: In Case You Missed it, Messaging Server 6.3 Now Supported on Sun Cluster 3.2.
Note also that only Messaging Server 6.3 supports Sun Cluster 3.2 software. Previous versions of Messaging Server do not support Sun Cluster 3.2 software.
BTW, if you're looking for what's supported for the Communications Suite 5 version of Messaging Server (and Calendar Server), here's the scoop.
