IPFilter and P3Scan

Hi,

This week, I 've installed P3Scan package on a FreeBSD 4.9R to filter
pop3 (for viruses) through my firewall.

The description of the P3Scan package explain how to use with ipfw, but
no with IPFilter.
P3Ssan listen on the 8110 port, so I've tried to do :
map external_interface internal_lan/24 -> external_address/32 proxy
port 8110 pop3/tcp
but ipnat doesn't like this rule :
1:ioctl(SIOCADNAT): No such file or directory

How can I do to filter transparently pop3 with IPF ?

Thanks
FAbrice