Your question is not clear to me...
Are you receiving smtp packets on the port you configured for utorrent?
In general you can drop smtp packets filtering the standard smtp port like this:
iptables -A <YOURCHAIN> -p TCP --dport smtp -j DROP
but if you are using the standard smtp port for utorrent, this will drop all packets.
There are some excellent HOW-TOs at www.netfilter.org