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SPF (Sender Policy Framework) and SRS (Sender Rewriting Scheme)
VisNetic Mail Server features support for Sender Policy Framework (SPF), an anti spam measure designed to fight email address forgery. SPF verifies the authenticity of the sender's FROM adress by performing a DNS query to verify that the sending server is authorized to send email on behalf of that address. To counter issues with SMTP forwarding VisNetic MailServer has introduced SRS (Sender Rewriting Scheme). SRS forces the rewriting of the MAIL FROM address by the forwarding agent.
DomainKeys technology is a proposed email authentication system for validating and proving authenticity of the domain of an email sender and also the message consistency and completeness. DomainKeys performs functions similar to SPF, because it stops forgery of email sender domains. But it can also ensure that the content of the email was not changed or altered in any way during the transmission. The messages are signed with private keys, the public keys are stored in TXT field of the domain DNS record.
Greylisting provides a "temporary reject" of any email not recognized by VisNetic MailServer. If the email is legitimate, the originating mail server will attempt to resend the message (per RFC) at which time VisNetic MailServer will accept it. The objective here is that most spammers or automated mailers are not configured to retry messages.
Razor2 is a distributed, collaborative, spam detection and filtering network. Spam is reported by users worldwide as it is received and a signature of a random part of the message is recorded in the Razor2 database. The signature is regularly re-created using a different random section of the message so spammers can not modify their messages to fool the system.
Email from unknown senders which cannot be clearly deemed spam or ham can be quarantined and inspected by the user, spam admin or server administrator, through WebMail or email client via Quarantine Reports or WebAdmin and Windows Administration Console's Spam Queue Manager and the sender Authorized, Message Deleted or Delivered once without authorizing (whitelisting) the sender. This allows for full manual spam processing by a person, semi-automatic by engaging Challenge Response authorization, or user-driven using the mentioned Quarantine Reports.
Bayesian filters statistically calculates the probability that each inbound message is spam. Because it is measuring probabilities, the Bayesian approach considers all the content in an email, both good and bad. The Bayesian filter-based engine is automatically trained and updated via a spam database that is uploaded to the server, and/or trained manually on your local network. Learn more.
SpamAssassin technology heuristically examines each message against hundreds of time-tested rules and checks. Each message receives a score based on the likelihood that it is spam. This filtering is completely automated, but spam-scoring thresholds are customizable. It also integrates directly with the Bayesian filter and offers many additional features, such as complete spam message reporting, header reporting and much more.
The optional Challenge-Response mechanism, the most foolproof spam blocking method known to block 100% of computer-generated spam. If the SpamAssassin and Bayesian filters cannot definitively prove that a message is or is not spam, a challenge message is sent to the message originator. The sender is presented with an embedded link in the message. Once the user clicks on this link and enters the displayed code, their message and all of their future messages will be allowed.
Additional Antispam Protection:
SURBLs (Spam URI Realtime Blocklist)
Unlike the use of RBLs, which filter on the From email address only, SURBLs allow you to filter and block messages based on spam domains that appear in message body URIs (usually web sites), the message bodies.
RBL and CF Bypass Files
VisNetic Mail Server supports several bypass files. If a named file exists with certain content, or if a message contains a recipient or sender listed in the bypass file, the content filters or anti spam filters will be ignored. The file can contain email addresses and domains, IP addresses, masks each per line.
A DNSBL is a service which provides a list of known spammers' IP addresses. The DNS server is used as a database or directory mechanism to store their IP addresses along with some other information. It can also be used as a DNSWL where the DNS server stores the IP addresses of genuine senders. In such cases you can use the DNS server for white listing and black listing purposes. DNSBL can reject messages before they actually enter the server.
Body Content Filters
Accurately filter 99% of all html spam messages using these sophisticated rule sets. A common spamming technique is to send messages in all HTML format, which prevents most message body scans from reading data correctly. These filters ensure that these messages are examined and spam is identified. HTML filters are updated with new VisNetic Mail Server software releases to reflect new HTML spam tendencies
This system is monitoring attempts to deliver messages to unknown users, to abuse SMTP commands, to send spam or to relay without authentication and when a certain threshold is reached, automatically closing further sessions for a period of time or permanently blocking connections from such IP addresses.
Command monitoring and service rate control
Maximum of bad commands, maximum of outstanding connection requests, exhausting command policies along with advanced rate controls effectively block mail bombing attacks.
Multiple RBL Servers
RBL (Real-time Blackhole Lists) allows checking of message senders against a number of RBL resources (such as spamhaus.org) for an indication of possible spam. A number of such servers can be found on the Internet which maintain lists of IP addresses of known spam-sources and offer them to others free of charge. Real-time means that the database is updated daily.
Blacklists, Whitelists, Bypass Files
Simplified Black and White List management allows users to block or authorize senders, domains and IP addresses. Bypass files allow messages to "skip" particular test for specified senders. White lists will skip the processing as a whole and immediately deliver the message. They can be defined at the level of server/domain/group/user and users can maintain their own whitelist through the WebMail interface.
Keyword lists are available as part of both Blacklist and Whitelist features. A message is then blacklisted or skipped from antispam processing based on its body content. Keywords significant to your business can be used to lower false positives rate.
Forbidden character sets filter
Often the best protection against unwanted email is rejecting email written in certain languages. The character set is determined from the message header. You can easily set charsets for all languages you do not want to accept, according to your region and languages in use in email communication.
Learn rules can be established for users to participate in increasing the accuracy of Bayesian filters and to automate the building of blacklists and whitelists. Any message sent to a special email address will be black or whitelisted, indexed by Bayesian as spam or ham, or removed from the list or index.
Spam Traps / Honeypots
Spam traps are email addresses exposed on websites to spambots, automated scripts collecting email addresses on the Internet, which will be subsequently used in spam campaigns. Email sent to a spam trap can be used in multiple ways: indexed to Bayesian spam database, block the sender's IP address for a period of time (Intrusion Prevention) or blacklist the sender IP for good. They can be configured to send a custom message to the sender (with usually invalid address though), or report his IP address by email to a collaboration antispam network of choice.
Processing of non-user accounts and outgoing messages
The AntiSpam module can work for all account types created within the server such as mailing lists, remote accounts, catalogues, to prevent abuse through accounts known to the public. Also outgoing messages can be scanned by AntiSpam, which is useful for ISPs who can't trust their users, but can be handy when a weak password is guessed by a spammer. Administrator can use Content Filters or Black and White Lists to achieve the desired behavior.