It's been at least 2-3 years since I used Qpopper, and I wrote a POP before SMTP script that monitored /var/log to do it... I don't recall it being that difficult though, and the usernames were automatically being logged...
Just noticed this though in the manual on Qualcomm's web site...
Quote:
log-login
When set, Qpopper logs successful
authentications using the specified
string. Within the string, an
occurrence of %0 is replaced with the
Qpopper version, %1 with the user
name, %2 with the user's host name,
and %3 with the user's IP address.
Default: none, unless
--enable-log-login used with
./configure, in which case (v%0)
POP login by user \%1\ at (%2) %3 is
used.
Did you compile it yourself? Maybe missed a option...