Посмотрите комментарии к main.cf
# JUNK MAIL CONTROLS
#
# The controls listed here are only a very small subset. See the file
# sample-smtpd.cf for an elaborate list of anti-UCE controls.
# The header_checks parameter restricts what may appear in message
# headers. This requires that POSIX or PCRE regular expression support
# is built-in. Specify "/^header-name: stuff you do not want/ REJECT"
# in the pattern file. Patterns are case-insensitive by default. Note:
# specify only patterns ending in REJECT (reject entire message) or
# IGNORE (silently discard this header). Patterns ending in OK are
# mostly a waste of cycles.
#
#header_checks = regexp:/etc/postfix/filename
А в файле filename прописывается что-то типа:
/*This is no SPAM!!*/ REJECT