Some lines CISCO-PIX manual
Some lines from CISCO-PIX515E wich explain the fixup command :
The fixup protocol smtp command enables the Mail Guard feature. This restricts mail servers to receiving the seven minimal commands defined in RFC 821, section 4.5.1 (HELO, MAIL, RCPT, DATA, RSET, NOOP, and QUIT). All other commands are rejected.
As of version 5.1 and higher, the fixup protocol smtp command changes the characters in the server SMTP banner to asterisks except for the "2", "0", "0" characters.