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BarricadeMX is a SMTP filtering proxy.
Developed by SnertSoft in collaboration with Fort Systems Ltd.,
BarricadeMX combines & enhances many of the most popular SMTP filtering techniques, along with
some new ones into a comprehensive all-in-one package that can be installed as the first
line of defense in front of any mail transfer agent, such as Sendmail, Postfix, Exim, Qmail,
and others on the same server or a separate machine.
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Mail messages have a standard structure and format that is covered by RFC 2045 et al.
"Multipurpose Internet Mail Extensions" (MIME).
A user's mail software is suppose to adhere to these standards for the formatting
and transmission of mail as 7-bit, 8-bit, or binary data. A lot of spam, in particular
those written in foreign languages, fail to adhere to these standards by sending
unusual, often unprintable, 8-bit character codes in messages that are only supposed to
contain 7-bit data for safe and correct transmission between mail servers. Many mail exchanges are
very forgiving or careless in what they accept and so this form of spam gets through. This milter
ensures that the content of a mail message adheres to the expected or declared
Content-Transfer-Encoding as described by the related RFC documents.
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Part utility, part filter. This milter collects the recipient addresses of messages sent
from white listed outbound senders. It can then optionally tag, reject, or discard mail from
unknown inbound senders, which haven't been white listed or previously cached.
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This milter implements a "call-forward" technique, which is similar to a "call-back"
(see below), but intended for use
by mail gateways that desire a method of verifying that the recipient of a message
exists on an authoritative mail store before the gateway accepts the message. Think of
it as a poor man's LDAP. Many
mail systems split the functions of mail transfer and that of storage & retrieval
over two or more systems. Historically a mail gateway would always blindly accept and forward
mail to their mail store, but spammers will often send mail to a domain using
a dictionary of user names, resulting in many error message returns that can sometimes
saturate the mail gateway. Often this situation is compounded by the mail gateway
queuing those useless error messages for days as they attempt to send them back to the spammers
that used throw away domains or mail servers that are now "off", eventually resulting in hundreds of
"double-bounce" errors being sent to the mail gateway's postmaster mailbox.
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This is a utility milter that can add to the recipient list of any inbound and/or outbound message one or more
blind-carbon-copy (Bcc) recipients depending on the MAIL FROM: and/or RCPT TO: addresses for any given message.
This is particularly useful for mail hosts that manage several domains that desire a simple and flexible
way to send a copy of messages to one or more mail boxes without the need for aliases, .forward files, or
similar.
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This milter provides an interface to the popular open source Clam Anti-Virus scanner.
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This milter provides a means by which client connection and envelope details and/or
message headers and content can be filtered using shell commands or scripts. Intended
as a way to implement quick & dirty content filtering solutions when there
is no other suitable milter available.
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A mail message contains several instances of date & time information, such as
when the message was originally written, possibly when it was resent, and when
each mail server en route handled the message. Spam messages often have incorrect
time-stamps, appear to be too old or too far in the future, and/or demonstrate an
inconsistent time-line.
This milter verifies that the date & time information within a message
is formatted according to RFC 2821,
that a message is delivered within a configurable time frame, and that
the transit of a message across mail servers reflects a consistent time-line.
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Count the number of SMTP messages failures from an SMTP client and tag, reject, or discard
subsequent messages if they exceed a given threshold.
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Grey-listing is a technique that uses the behaviour of a
standard-compliant mail server to temporarily delay the acceptance of mail. When a sending
mail server initially contacts a mail exchange to deliver a message,
the details of the message are
recorded and the
mail exchange signals the sending mail server that the message is
temporarily rejected. A normal mail server will place temporarily
rejected messages into a retry queue and after an appropriate delay
attempt to resend the message to the mail exchange.
Your mail server accepts the message later, when the sender retries.
The underlying principal here is that spammers
use "mail cannons" to send as much mail as fast as they can and so will
not implement a retry queue.
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A utility milter that can reject mail according to different message size limits per IP, domain name, or sender address.
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Message limit accounting is a facility to control the number of messages that traverse
a mail exchange according to domain, sender, or recipient. It could be used on the outbound
side like Hotmail's daily message limits to limit local user's consumption
(particularly if they appear to be infected by a mass mailing worm);
it could be used inbound as an alternative to grey-listing.
It could be enabled and disabled as needed during periods of peak mail activity
such as during a virus outbreak or spam holiday season.
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Extracts URLs from a mail message and checks it against one or more URI blacklists.
Can also verify if any link is bad.
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A utility milter that adds a report header to each message containing the
sender's NS records, suitable for downstream processing.
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A mail filter to help with DSN backscatter and null address abuse.
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A sendmail / p0f interface that allows for passive OS finger-printing
analyses of SMTP connections and adds a report header to each message for future
action by downstream filters.
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A utility milter that records all sender & recipient pairs that are rejected
by Sendmail or other milters in order to send peroidic reports to the recipients.
Useful for people who are worried about loosing mail, because of anti-spam filtering and
would at least like to know what email addresses were rejected just in case one of them
was legitimate.
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The Server Index Query (SIQ) protocol is intended to provide a
standard means by which a mail exchange can query one or
more third party services for a score based on facts, trends,
or reputation of a connecting mail server's IP address and/or
the sender's domain. This milter is a client side implementation of the
current Internet draft.
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This milter implements a "call-back" technique. A mail exchange
that is processing an inbound SMTP transaction looks up, via the domain name
system, the mail server responsible for the sender's mail. The mail exchange then
opens an SMTP connection back to the sender's mail server and emulates
an error return message to the sender without actually completing
the transaction. The mail server being queried normally accepts
or rejects the sender's mail address in the early stages of the transaction.
The idea here is that spammers use a variety of false and often invalid sender
addresses in the SMTP transaction, such as false nonexistent domains,
randomly generated user names from well-known domains, facade mail systems that
don't accept any mail, throw away mail boxes that fill up with errors and
replies to unsubscribe, etc.
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This milter provides an interface to the
popular open source SpamAssassin mail analysis tool, which returns a score based on
a comprehensive set patterns and Bayesian checks.
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This is an independent implementation of the Sender Policy Framework (SPF-Classic),
This mail filter will accept, tag, reject, and/or discard email according to a domain's
published SPF records, which document the valid sources of mail for the domain.
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This is an SMTP multiplexer, which takes the input from an SMTP client connection and copies it to one or
more SMTP servers. Intended as means to debug and test different mail server configurations using a
production mail server's live data stream.
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s8 = Sendmail 8.13+
p = Postfix 2.3 w/ milter support
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? = untested, expected to work
* = untested, may require updates
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