No, actually, you can't...
The number part is linked by the IE cache system to how MSN Messenger wants to store it,
however... MSN hashes the DP information before using it to save and look up the cache. (hashing is a one-way encryption).
Thus there is no way of knowing which DP belongs to whom.
You can however resolve a DP object (<msnobj .... />) to a cache file, but you'll need more information than just the email address