You are starting sendmail to build the aliases file (which it does
per line 2).  It looks up each interface IP to figure out who it
is.  You have an IP that doesn't resolve.  Best is to fix that.
/me wonders about suppressing that lookup for newaliases.
Will ping the org folk.

Quoting jeffrey@jeffreyf.net (jeffrey@jeffreyf.net):
> When I run the 'newaliases' command on my home NetBSD box, currently
> named testbsd.org, I get this alert:
> 
> Dec 13 15:33:17 testbsd sendmail[8003]: gethostbyaddr(192.168.1.100) failed: 1 
> Dec 13 15:33:17 testbsd sendmail[8003]: alias database /etc/mail/aliases rebuilt by root
> 
> Is this simply a result of using dhcp and that my machine not does not
> have a hostname on the Internet domain name system?