In /usr/lib/X11/app-defaults/XBiff:

*checkCommand:/usr/local/bin/checkmail

In /usr/local/bin/checkmail:

#!/bin/sh
ls $HOME/Maildir/new > $HOME/.curmail
if [ "`wc -l $HOME/.curmail | colrm 8`" != "      0" ]; then
    if [ -f $HOME/.prevmail ]; then
        if [ "`diff --brief $HOME/.curmail $HOME/.prevmail`" ]; then
            mv $HOME/.curmail $HOME/.prevmail
            exit 0
        fi
        exit 1
    fi
    mv $HOME/.curmail $HOME/.prevmail
    exit 0
else
    exit 2
fi