<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML ><HEAD ><TITLE >Blocking Access to Other SMTP Servers</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="Spam Filtering for Mail Exchangers" HREF="index.html"><LINK REL="UP" TITLE="Considerations" HREF="considerations.html"><LINK REL="PREVIOUS" TITLE="Multiple Incoming Mail Exchangers" HREF="multimx.html"><LINK REL="NEXT" TITLE="Forwarded Mail" HREF="forwardedmail.html"></HEAD ><BODY CLASS="section" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Spam Filtering for Mail Exchangers: </TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="multimx.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 3. Considerations</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="forwardedmail.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="section" ><H1 CLASS="section" ><A NAME="otherservers" ></A >3.2. Blocking Access to Other SMTP Servers</H1 ><P > Any SMTP server that is not listed as a public <A HREF="gloss.html#mx" ><I CLASS="glossterm" >Mail Exchanger</I ></A > in the DNS zone of your domain(s) should not accept incoming connections from the internet. All incoming mail traffic should go through your incoming mail exchanger(s). </P ><P > This consideration is not unique to SMTP servers. If you have machines that only serve an internal purpose within your site, use a firewall to restrict access to these. </P ><P > This is a rule, so therefore there must be exceptions. However, if you don't know what they are, then the above applies to you. </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="multimx.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="forwardedmail.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Multiple Incoming Mail Exchangers</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="considerations.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Forwarded Mail</TD ></TR ></TABLE ></DIV ></BODY ></HTML >