05-10-10 | Blog Post

How to Filter Spam, Phishing Emails, and Viruses Correctly without Blocking Legitimate Mail

Blog Posts

Email is an essential service especially in today’s world of convenience and quick communication between clients and businesses. However with email come viruses, phishing schemes, and spam that can be a major drawback to hosting your own email system. Although these malicious messages can never be completely avoided, there are many different services and tools that when implemented correctly can reduce the number of spam to controllable amounts.

Despite the ability of these excellent programs to prevent you from seeing a million nasty messages they can also impeded the successful delivery of legitimate mail. Most often when I am configuring a server or troubleshooting an email delivery problem for a client I find that there is very little knowledge of the spam policies used by other email providers.

The most troublesome and often the hardest to find any information on email policies are web based email service providers such as Yahoo, Microsoft Hotmail, and AOL. These services often use a variety of methods to determine if a message is legitimate and should be passed through or spam which often will be simply deleted.

Another major problem with spam filters is that they will often automatically block the address of your mail server if they receive a specific number of spam messages. There are several things you can do to prevent lost email as well as sudden delivery issues. The first thing that most email services require is a reverse DNS record that matches the forward record for the mail server.

For example, if I have one mail server at mail.example.com that points to IP, I will also need a reverse entry that point to mail.example.com. This allows the receiving mail server to do a trace back to the sender. Often spam machines will not have a reverse DNS since they do not want the authorities tracing them back to their source.

Another essential method is to add an SPF or Sender Policy Framework entry in to your DNS zone. SPF records allow other servers to do a quick lookup and determine if the source address of a message is being spoofed. The record itself specifies which hosts within a domain are allowed to send email.

The final step is to ensure that your email server is not an open relay, which means that the server will only relay messages from servers that you know and trust. Most mail software will have options that can be adjusted to allow relayed mail from specific hosts. For more information on email privacy and security please check the following links.

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