Sometime Today, Kishor Bhagwat assembled some asciibets to say:
tcpd can work for UDP also. There is no reason why not. A request comes in, tcpd validates it, and calls the appropriate executable.
udp has no source address, so although tcpd will work, there's no way it can do source ip based filtering.
Also, why not mention smtp in the inetd configuration file for incoming requests, and call sendmail?(though that wont achieve much,
sendmail -bs in /etc/inetd.conf.
Philip