This really isn't a sendmail issue but you probably don't have the POP3 daemon installed. On Red Hat Linux, it's part if the IMAP RPM (IMAP-4.5-4). Use RPM to check and see if it's installed using the command rpm -q imap. If you get a null response it's not installed and you need to install it to get this to work. Just mount your CD-ROM using the command mount /mnt/cdrom and install the IMAP RPM using the command rpm -Uvh /mnt/cdrom/RedHat/RPMS/imap-4.5-4.i386.rpm. Try connecting from your clients again. If you still have a problem have a look at your /etc/inetd.conf. Look for the line which ends with ipop3d and make sure it's not commented out (e.g. prepended with a `#'). If it is, remove the pound sign, restart inet using the command /etc/rc.d/init.d/inet restart and try connecting again. Unless some step discussed above was missed, that should take care of the problem.