It's not a restriction; it's just basic system administration 
practice. If a host has any possibility of emitting E-mail with its 
domain name in the return address, then there must be an A or MX 
record in the DNS, pointing to an SMTP server, so that replies can 
come back. Failing to do so is rude, and indicative of either 
ignorance of or disregard for best current practices of the Internet.

	Erik <fair@netbsd.org>