As SteveO86 stated, you have 2 options
-Turn on or turn off the Enterprise-Only Message Icon
This needs to be done by the BES admin and will happen on every handheld conected to that BES.
2. Download install and purchase Empower BES
This will basicaly do the same as above, but only happens to you, not everyone on the BES.
Now in both instances, the main MESSAGES icon will still always get ALL messages... just hide it and ignore it.
In both instances you will have a BES mail icon and your BIS mail icon(which I assume you already have that)
Hope this helps