Why do I get "Unsupported major.minor version 49.0" when starting JBMS?

versions: 1.0M3, 1.0M4

You have installed the JDK 1.5 version on JDK 1.4.  Milestone 3 provides two versions.  See the Milestone Releases Page for details.  You should upgrade to JDK 5.0 (aka 1.5)  as soon as possible see this page for details.

JBoss Mail Server 1.0M4 requires JDK 5.0 (aka JDK 1.5).