why does sendmail default to c18-ss-1-lb.cnet.com when domain is unknown?

[Bazooka Joe]

Sry for a OT sendmail question

In my mail queue I have a bunch of email w/ the domain
c18-ss-1-lb.cnet.com and the status is "Deferred: Connection timed out
with c18-ss-1-lb.cnet.com."

Each of the email the user mistypes the domain so that it is not a
resolving domain name. Sendmail replaces the invalid domain with a
cnet address.  ex thutrang at c18-ss-1-lb.cnet.com.

The confusion comes when the user gets the email bounced back after 5
days w/ the domain replaced w/ the cnet one.  Then they complain to
me.

thx
bazooka