SPAM Prevention

With proper setup and maintenance, mail servers will generally accept mail from legitimate sources only. Otherwise, a mail server will accept mail indiscriminately - including Spam.

VisNetic MailServer offers a number of security features that directly deal with Spam. All of these features can be used in conjunction to help prevent Spam:
  • Tarpitting
  • Content Filtering
  • Spam Blocking
  • Deny telnet
  • Real-Time Blackhole list
  • Verify MX
  • Reverse DNS


Tarpitting (recommended)
This feature is very effective against Trial and Error Harvesting. Administrators can configure VisNetic MailServer to check for unsuccessful attempts to deliver messages to unknown users by external or local non-existing users. If the number of attempts in a session exceeds the number specified by the Admin, the IP address of the sender will be remembered for that period and no access from that IP address will be allowed within that period. We recommend a count of 3-5 and a period of 60 to 240 min.

Content Filtering (recommended)
This feature normally requires the most management, but can be the most effective against Spam. You can filter incoming and outgoing emails based on a number of variables including, subject, sender and/or recipient, key words in the text, and more. You can also create filters that Reject, Accept or Delete messages that match specific content.

Spam Blocking: Global, Domain, and User Levels (recommended)
This feature is very effective against spammers that you can identify. You can filter based on Domain, IP address, or Email address. This feature enables SPAM blocking services that incorporate a black list of known spammers to effectively block the relaying of SPAM through VisNetic MailServer. You can also block ranges (this applies to domains and email addresses as well).

Real-Time Blackhole list (not recommended for heavy traffic servers or dialup connections)
This feature can be extremely effective when used to block a lot of Spam with little or no management. This feature checks a third party database of Known Spammers. These third party services keep updated databases of Spammers that have been reported to their service.

Deny Telnet (recommended)
Telnet sessions are sometimes used to harvest email addresses. When you disable the Telnet sessions, the mail server will not accept Telnet commands. A user will be able to establish a connection using telnet, but the server will not accept any commands once the session is established.

Verify MX (not recommended for all users)
Another security check is to receive mail only if the sender has a valid MX record. Checking to see if the sender's domain has a Mail Exchange DNS record is one way of doing this. If checked VMS will perform a MX lookup and reject the mail if no MX record exists. If you have incorrectly configured your DNS server this may cause harm. Use the DNS Query tool to check your DNS settings.

Reverse DNS (not recommended for all users)
This security option check is to receive mail only from servers that have reverse DNS PTR record (rDNS).