SMTP SPF Filter
  for Microsoft Windows 2000/2003/XP IIS and
  Microsoft Exchange 2000/2003
  by Michael R. Brumm

Update (2004/09/06)

GFI Software has obtained the rights, source code, and copyright to this SPF filter software and has integrated its functionality into their award winning MailEssentials software. The SPF filter feature is still available as freeware.

I would also like to add that I personally use full versions of both GFI's MailSecurity (for virus scanning) and MailEssentials (for spam filtering) on my own Exchange server. GFI provides freeware versions of both of these, and 30-day or 60-day evaluation of the full versions. For more information, visit:
GFI MailEssentials  and  GFI MailSecurity

Newest SPF Filter Available in GFI MailEssentials

You can download the beta of GFI MailEssentials version 10 with the SPF filter feature from:
    ftp://ftp.gfi.com/beta/

There seems to be more than one build (me10_spf.exe, me10_spf_20040902.exe, ...). I suggest you download the newest.

NOTE: If you have a previous version of GFI MailEssentials, uninstall it first (your settings will be preserved).

If you have questions about SPF, visit:
    The SMTP+SPF: Sender Policy Framework Web Site

If you have questions about the SPF filter, visit:
    The GFI MailEssentials Forum
 

How to Add an SPF Record in Windows 2000 and Windows 2003 DNS

How to Remove the Old (non-GFI) SPF Filter

For a quick, clean, and error-free uninstall of the SPF filter, follow these steps:

1. Disable all SPF filters using the MMC plug-in
2. Close all MMC windows
3. Stop the "IIS Admin" service
4. Uninstall the SPF filter and the MMC plug-in
5. If the uninstall hangs, refresh the services and see if the "IIS Admin" service has started again. If it has, stop it again. The uninstall should then complete.
5. Restart the "IIS Admin" service and all services that were stopped during step #3 (or just reboot the computer, which is easier)

Sorry about this extra work, but it is caused by the "IIS Admin" service hanging onto unused event sinks (thereby preventing removal), and working around this problem is not a priority.