Anti-spam features
1. Directory Harvest Attack protection
Directory Harvest Attacks abuse the design of
the SMTP protocol. When one mail server attempts
to deliver a message to another mail server, it
first inquires whether the recipient address exists
on the target mail server. If it exists, the target
mail server sends a confirmation; if not, the
server responds with negative answer. Spammers
take advantage of this communication by submitting
multiple requests with different combinations
of first and last names. By harvesting valid responses,
spammers get very precise list of valid email
addresses suitable for future spamming.
To fight directory harvest attacks, Kerio MailServer
tracks the number of failed recipients during
a conversation with the sending mail server. If
a predefined threshold is met, the offending mail
server will be blocked for a period of one hour.
2. Spam Repellent
Spam Repellent helps fight off spam zombies and
viruses by delaying replies during the SMTP handshake.
Since spam mailers and viruses are very impatient
when it comes to SMTP communication, they don't
wait if the replies from a target mail server
are not timely, and simply move on to try another
target. By injecting an artificial time delay
of a couple of seconds into the SMTP handshake,
Kerio MailServer may filter up to 60-70 percent
of spam and viruses without ever receiving the
messages for analysis.
3. SpamAssassin
SpamAssassin is a powerful anti-spam engine integrated
into Kerio MailServer that identifies junk email.
Combining several technologies, SpamAssassin ensures
that bulk spam email is properly captured and
legitimate email is reliably delivered.
Heuristic engine
Using the heuristic engine in SpamAssassin, each
email message is analyzed against multiple anti-spam
rules and gives it a numerical rating based on
which rules were matched. If the message rating
exceeds the spam threshold, the message is marked
as spam and then filtered according to the user's
wishes.
 Bayesian
filter
To further increase the success rate and tailor
the spam filter to the individual needs of every
company, Kerio MailServer allows Outlook and WebMail
users to easily train the Bayesian filter in SpamAssassin
for every email message that is miscategorized.
The Bayesian filter can also be trained by dragging
and dropping email in or out of Kerio's Junk E-mail
folder in Apple Mail or any other IMAP email client.
SpamAssassin then compares incoming messages to
spam messages reported by other users, and sees
how similar they are to those reported messages.
Underlying technology
SpamAssassin is an anti-spam technology from the
open-source Apache SpamAssassin Project.(www.spamassassin.org).
4. Dual filtering policies
Dual filtering policies give administrators the
ability to tag messages with low spam score as
spam and forward them to user's Junk Email Folders,
whereas messages with really high spam score can
be automatically discarded without ever reaching
user's mailbox. Discarding emails with high spam
scoring can help save hard drive space and reduce
the amount of emails that endusers need to audit.
5. SURBL blocking
Similar to RBLs, SURBL (Spam URI Realtime Block
Lists) will block messages that contains URIs
(usually Web sites) in the message body that links
to known spam hosts.
6. Real time blacklist support
Checks sender's IP address for each incoming SMTP
connection. If the IP address is in the database
of open-relay servers identified as spammers,
the incoming email is refused.
7. SPF verification
Just like Caller ID, SPF (Sender Policy Framework),
an emerging standard for domain authentication,
helps counter forged "From" addresses
in email. Using SPF in Kerio MailServer provides
better guarantee that the email came from the
domain that it asserts it came from.
SPF requires the owner of an Internet domain
to modify DNS records and specify which servers
are authorized to transmit e-mail for that domain.
8. Microsoft Caller ID verification
Using Microsoft Caller ID anti-spam technology,
Kerio MailServer checks whether an incoming email
is actually being sent by the authorized mail
server for that domain (hence the name, Caller
ID). Kerio MailServer will decline all email messages
with forged "From" addresses.
As a side effect, publishing own Caller ID also
helps companies protect their reputation by eliminating
the possibility of having their domain name hijacked
for spam purposes.
9. Sender's domain verification
Rejects emails when sender's email address is
using an invalid or fake domain name.
10. Custom filtering by address, subject,
content or size
Can reject emails from notorious spammers not
known to public blacklists. Allows for the creation
of a custom blacklist. An efficient measure to
fight against empty "From" fields in
spam emails.
11. SMTP authorization by user name and
password
Requires users to authenticate before sending
email through SMTP.
12. SMTP authorization by IP address
Limits SMTP relay access to specified range of
IP addresses. Relay access is usually allowed
for users within local area networks (LAN).
13. "Sent email per hour" quota
Limits number of emails that one user (IP address)
can send within a specific time period. Significantly
reduces the risk of mail server abuse if spammer
has accidentally acquired trusted user access
rights.
14. "Concurrent connection"
limit
Limits number of concurrent SMTP connections made
from one IP address (one user). Protects mail
server resources by limiting mail load. Prevents
the use of specially designed spam programs that
create multiple connections to increase the number
of emails being sent to SMTP server.
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