I removed a bunch of command line switches from zmlmtpinject in the 6.0 codebase. I believe that this is stale code that was used internally for performance testing. If you are using any of these features, please let me know so we can figure out how to best accomodate you in 6.0:
-w, --warmUpThreshold
-S, --stopAfter
-u, --username (this is not sender/recipient)
-D, --domain
-z, --repeat
See bug 21111 for the details and proposed new CLI.