Two-tiered Spam Protection: GDMilter and SpamAssassin

GDMilter is a sendmail milter designed to help ensure that mail received to the server actually originated from an authentic mail server, not a bot or other mass mailer. When GDMilter receives a piece of mail, it returns a delay to the sender mail server. If the sending server is actually a mail server, it will retry the message. If it is not a mail server, but rather a bot or similar, then the mail is not retried. If the message is retried then the sender is whitelisted as an authentic mail sender. The main advantage to this method is that a filter isn't processing the entire message on the server, using up system resources. Any sender/server that is already known to be safe can be manually whitelisted to bypass this delay process.

SpamAssassin is the second tier of defense. If spam passes the first test, SpamAssassin then scans the message based on content and gives it a score. If the score is over the configured threshold (0-40), it is deleted. Messages marked as spam can also be placed in a single file on the server, however this file can grow rapidly and will need to be monitored/rotated on a regular basis. Whitelisting and blacklisting can also be done via SpamAssassin.

SpamAssassin and GDMilter will provide great improvements in spam prevention. They are not designed to solve all the server load problems that may be occurring on very busy servers. By nature of GDMilter, is is a memory resident program, drawing its resources for your server's RAM memory. Because it is memory resident program, it can not be run effectively on any server with less than 1 GB of RAM and any CPU that is less than 1.4 GHZ. As such, you cannot install this GDMilter/SpamAssassin solution on any server without these minimum hardware requirements.

Why is my mail delayed?
The milter is designed to delay mail. The delay is set to 1 minute. Under normal circumstances mail is only delayed a minute or two until the from/recipient/mail server IPs are whitelisted. After being whitelisted there is no delay at all. However, if the sending server is very busy, there is the potential for a longer delay. If you have IPs for yourself or your clients that are known to be "spam safe" those can be whitelisted. This will cause anything sent from those IPs to bypass the milter process. It is not wise to whitelist a receiving domain, as it defeats the purpose of having a filter on the server to cut back on sendmail processes, mail traffic, and spam, however, it can be done with whitelist interface.

Why am I still getting spam?
You can turn down the required spam score for spamassassin the default of 5 to a lower setting and this will filter out more spam messages. Please note that this also creates a higher chance that legitimate mail will be blocked too. GDMilter and SpamAssassin will not stop all spam, as any spam solution cannot do. The products will reduce the amount of spam that reaches the end user.

Why am I getting so many support requests after installing this product?
The initial adjustment period that is associated with these products can cause some clients to worry about lost or delayed mail. In most cases, after users learn what was going on and see the positive affect of less spam, most support requests decline within a week of implementing the filters. In order to keep any surge in support requests to a manageable level, we advise that you spread out the timeframe over which you activate the services on each of your servers. Installing the spam services on one or two servers a day is advisable