Home > Articles > Security > Network Security

📄 Contents

  1. E-mail Hazards
  2. Getting the Upper Hand
  • Print
  • + Share This
Like this article? We recommend

Like this article? We recommend

A number of tools are available for the harried mail administrator. Thinking of E-mail in terms of postal mail, it's straightforward to classify these filtering tools according to whether they "open the envelope." In other words, some tools care only about what's written on the outside (information about the sending mail server and the SMTP mail delivery instructions), while others perform a thorough examination of the mail's actual contents (the mail headers and body).

Envelope Tests

When one mail server sends mail to another, a few vital pieces of information are exchanged:

  • IP address of the host connecting to the receiving mail server

  • Sender's (supposed) hostname

  • Sender's (supposed) E-mail address

  • List of recipient E-mail addresses

Before the receiving mail server agrees to accept the rest of the mail, it has a chance to use this bit of information from the "outside of the envelope" to decide whether it should reject the mail outright. A few rudimentary tests can be performed:

  • Is the connecting host on a DNS block list?

  • Are the mail's recipients actually valid users?

  • Is the connecting host "authorized" to send mail from its domain?

Let's take a look at each of these types of tests, and what they accomplish.

DNS Block Lists (DNSBLs)

The IP address of the connecting client can be looked up against various DNS-based lists of spammers, spam sources, and spam-related sites. These lists serve as an informal "reputation rating" for Internet hosts (from individual IP addresses to blocks of IP addresses to entire ISPs or domains), in much the same way that credit bureaus let creditors get a feeling for whether a potential customer is likely to make his payments. IP addresses that become known for sending out a lot of spam end up on these lists, which are generally available for any mail administrator to consult.

The purpose of block lists is twofold:

  • Protecting one's own network from receiving mail from sites that are known to send spam

  • Punishing those sites in the hope of encouraging their administrators to reform their ways

The power of these lists, for better or worse, is achieved when large numbers of sites use them; as more sites use a given list, more parts of the Internet refuse mail from listed sites. This strategy can be crippling to a listed site, isolating it from much of the Internet. When a listed site meets certain "cleaned up" criteria laid out by the operator of the particular block list, it can be removed from the list.

Block lists are not without controversy. The fact that each block list operates by its own rules and sets its own criteria for listing and delisting a site means that mail administrators need to be careful about which ones they choose to use and trust. Some are more aggressive than others, more willing to list large domains—for example, all of AOL.com. You might imagine that blocking all mail from so many millions of users can have a crippling effect on your own users who need to talk to people who are AOL subscribers. Since a certain amount of human judgment is involved in the listing process, innocent sites can also end up on these lists, and some block lists are slower than others at removing sites.

That said, most DNS block lists are quite effective; the more reputable lists, such as the Spamhaus Block List, are in widespread use these days. Their effectiveness is not lost on spammers, who have targeted several DNSBLs with distributed denial-of-service (DDoS) attacks over the past year. By their very nature as DNS-based lists, DNSBLs are vulnerable to such attacks, and two of them—Osirusoft and monkeys.com—were forced to shut down last year as a result of prolonged DDoS attacks.

Tests for Valid Recipients

It seems like common sense to suggest that your mail server should only accept mail addressed to actual users it knows about, but an amazing number of mail servers out there aren't doing this relatively simple check before accepting mail. Instead, they accept the mail and only later in the process discover that the recipient doesn't exist, forcing the mail server to then bounce an error message to the (supposed) sender.

Where this causes serious problems is with "dictionary attacks," used by spammers to try to find out which E-mail addresses actually exist at your site. When a dictionary attack occurs, the spammer sends mail to a large number of addresses that start with common names (jack@..., jackie@..., jacklyn@..., jackson@..., and so on) in the hope that some of these won't result in a bounce-back message. The ones that don't bounce, of course, are valid addresses that the spammer can add to his list. In the meantime, forcing your server to accept this mail in the first place can seriously slow that server during a dictionary attack, when it might well be faced with thousands of such "tests" an hour.

Fortunately, most modern mail servers have built-in features that allow you to explicitly list the E-mail addresses for which you want to accept mail, and automatically reject mail destined for other addresses (for example, Sendmail's "virtusertable" feature).

SPF: Sender Policy Framework

In theory, mail servers should be receiving mail only from local mail clients, and from other mail servers that are "authorized" mail exchangers (MX'es) for their domains. This is how the Internet E-mail system was designed to work, but this intention is not strictly enforced. Knowing this, spammers often take advantage by connecting directly to mail servers to send their spam from dial-up accounts and broadband-connected hosts—hosts that are not listed as "official" mail servers for their domains.

Letting just anyone connect to your mail server generally leads to bad things. If your mail server is poorly configured, it might well agree to deliver mail to anyone from anyone, whether the recipients are local or not. That's called an "open relay," one of the spammers' favorite tools for broadcasting their mailings at your expense. Even if your server is secured against relaying, though, spammers connecting directly to your mail server can use that connection to deliver spam to your local users. Ideally, you should accept mail only from clients and servers that are properly authorized to send your server mail.

Sender Policy Framework (SPF) is an attempt to finally start enforcing this longstanding convention. It works by having administrators add a bit of extra information to the DNS records for their domains, listing the hosts that are authorized to send mail from that domain. When your mail server receives a connection from a client, you can check its IP address against the SPF information in the DNS record for the sender's supposed domain. If the client's IP address is not in that list, it's not an "authorized" mail server for that domain, and should therefore be refused a connection. If there's no SPF information in the domain's DNS record, however, no conclusion can be drawn.

Before SPF will be effective, it needs to be adopted on a reasonably large scale. Administrators need to add SPF information to their DNS records, and mail server software needs to be updated to incorporate SPF support. On the other hand, AOL has been testing SPF, and has plans to implement this technique in 2004. When that happens, AOL will reserve the right to refuse E-mail from sites that don't have SPF records, and that may well be the incentive that administrators across the Internet need to get onboard, given the sheer number of AOL subscribers.

And That's It for Envelope Testing

So now you're familiar with the many hazards that lurk in the average user's E-mail inbox, as well as the technique of the "envelope test," where a number of quick checks can help to cut down the amount of spam your mail server bothers to process. These techniques alone can save a significant amount of bandwidth and CPU cycles—not to mention user and administrator sanity—in the battle against spam. As this series continues, you'll learn about more of the tools available when fighting against this mail administrator's nemesis, and then we'll get into the meat of the thing: how to put all of this knowledge to use.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020