Instigates lawsuit against the vendor for having shipped a product with bugs in it in the first place. Well, it's upgrade time. Should take a few hours. Rewrites sendmail.cf from scratch. Hacks kernel to implement file locking.
Rewrites sendmail in assembly.
Hacks kernel to . . . Puts mail use policy in motd. Locks accounts that go over mail use quota. Keeps quota low enough that people go back to interoffice mail, thus solving problem. I've got my boots and backpack.