Ah! The joys of Sendmail cf configuration. How arcane can the following get?
R$+ $@ $>93 $1
That line tells Sendmail to masquerade the headers of an email as another domain. Luckily I just generated another sendmail.cf with my install-sendmail script and did a vimdiff of both files. Comments are helpful too, and takes some of the bite out of the .cf file.
