This is a fork of mailbrute.c, a program that launches a "RCPT TO:"-based dictionary attack on an SMTP server in an attempt to enumerate usernames.
The original mailbrute.c was written in 1999 by axess (axess@mail.com) of the old b0f (buffer0verflow) security research group. Since 2000 it has lived at http://packetstormsecurity.com/files/author/231/, and since 2003 a copy has also lived at http://examples.oreilly.com/networksa/tools/ because it was referenced by the O'Reilly book "Network Security Assessment".
Changes:
2013-09-22: [mjb2010] Only print the "Recipient ok" response, not the whole 500-char receive buffer.