##########################################################
# Defense in Depth - Anti-SPAM for sendmail environments #
# by David Bank --> http://dave.trianglenug.org          #
##########################################################
# Code Example #3 - Perl - MIMEDefang - filter_sender()  #
##########################################################


###############################
# Declare a hash of hosted Domains
#	- Key: Domain Names we host
#	- Value: A flag as to if the Domain should be
#			receiving E-Mail (0=No, 1=Yes)
###############################
%OurDomains=(	"domain1.tld", 1,
		"domain2.tld", 0 };

[...]

sub filter_sender
	{
	# Read parameters passed to function
	my($sender, $ip, $hostname, $helo)=@_;

	# Local variables
	my(@chksender);
	my($chk, $chksenderdomain);

	# Search the list of our hosts using the $ip argument
	if ( exists($OurHosts{$ip}) )
		{
		# The connecting host is our own host, don't bother checking further
		return('CONTINUE', 'ok');
		}

 	# The sender cannot claim to be a user in any Domain we host since we have a
	#	already eliminated the sending host as being ours

	# Make sure the address we're checking is in all lower-case
	$chk = lc($sender);
	# Remove any angle-brackets from address
	$chk =~ s/^<//;
	$chk =~ s/>$//;
	# Split the Sender address into Address and Domain Name
	@chksender=split(/\@/, $chk);
	# Extract just the Domain
	$chksenderdomain=$chksender[1];
	if ( exists($OurDomains{$chksenderdomain}) )
		{
		# Reject connection - fraudulent Foreign sender claiming to be local
		return('REJECT', "FRAUDULENT ADDRESS: $helo is not $chksenderdomain");
		}
		# End of IF
	}
	# End of filter_sender