The sendmail program automatically takes advantage of DNS lookups or MX records to resolve addresses and canonical hostnames. If your site is a UUCP-only site (or is otherwise not connected to the Internet) and does not run named(8) locally, you should probably disable NAMED_BIND:

APPENDDEF(`confENVDEF', `-DNAMED_BIND=0 ')
                                      
                            disable DNS lookups

If you are running a precompiled sendmail binary, you can use the -d0.1 debugging command-line switch (-d0.1) to determine if NAMED_BIND support is included (if it appears in the list, support is included).