Fighting Spam and Viruses at the Server, Part IV
- Tweaking Your Exchange and DNS Servers
- Add-On Spam-Fighting Tools for Microsoft Exchange Server
- Spam-Blocking Appliances
- Setting Up an Inexpensive but Powerful Spam-Fighting Box
- Spam-Blocking Services
- Wrapping Up
In parts I through III of this series, we looked at the problems mail users (and their administrators) face due to spam, and techniques available to mail administrators to help in dealing with this continually growing threat. Now we can finally look at the tools and tricks Windows mail administrators can use to get their incoming mail services back under control. These items can be sorted into three basic categories: Microsoft Exchange tricks, add-on software tools, and a new class of Internet appliances.
Tweaking Your Exchange and DNS Servers
Which spam-fighting features your Exchange server supports depends on what version of Exchange you're running. Exchange 2003 supports both DNS block lists (DNSBLs) and blocking email sent to invalid accounts, without needing any external tools to handle these features. Older versions of Exchange don't support these features without external add-on software. You can change these settings with the Exchange Server System Messenger:
For DNSBLs, select Global Settings, Message Delivery, Properties, Connection Filtering and use the Help information if needed to complete the setup.
To block email to invalid accounts, use Global Settings, Message Delivery, Recipient Filtering.
Your Microsoft DNS server also offers at least one aspect of SPF support, as discussed in part I of this series. In both the Microsoft Domain Name Server 2000 and 2003, you can add a text field, which allows you to create an SPF record that other people's servers can check against. Setting this value at least will make it easier for your mail to reach others, as SPF gains ground in the mail transit world. To add this field, open Administrative Tools, DNS Console. Open the zone in which you want to add the SPF record, select Other New Records, and choose Text (TXT) from the type list.
This is the most basic form of a useful SPF record:
example.com. TXT "v=spf1 mx -all"
where example.com. represents your domain and extension (note the period [.] at the end, which is required), and the remainder says that only the machines matching your MX records send mail from your domain. Mail coming from any other machine on your domain is then refused by mail servers that are configured to use SPF records. For more on SPF syntax, see http://spf.pobox.com/dns.html.
Microsoft Exchange doesn't yet support SPF as a spam-defense tool, but at least you can prevent your hosts from being used as spam generators by adding the DNS entries.