According to Symantec, the Sober Worm then uses its own SMTP engine to send itself to all the email addresses it finds. The subject in the email may be any of the following. . .

The Sober worm is a highly-damaging email worm that spreads by emailing itself to all addresses in a user’s email address book, for which the Sober worm uses its own on-board SMTP (Simple Mail Transfer Protocol) engine.