Email filter techniques
The FortiGate unit has a number of techniques available to help detect spam. Some use the FortiGuard Antispam Service and require a subscription. The remainder use your DNS servers or use lists that you must maintain.
Black white list
These are the types of black white lists available. They include:
The FortiGate unit compares the IP address of the client delivering the email to the addresses in the IP address black/white list specified in the email filter profile. If a match is found, the FortiGate unit will take the action configured for the matching black/white list entry against all delivered email.
The default setting of the
smtp-spamhdripCLI command is
disable. If enabled, the FortiGate unit will check all the IP addresses in the header of SMTP email against the specified IP address black/white list.
- Email Wildcard
The FortiGate unit compares the sender email address, as shown in the message envelope MAIL FROM, to the pattern in the patterned field. The wildcard symbol is used in the patterned to replace the characters in the address that may vary from the pattern. If a match is found, the FortiGate unit will take the action configured for the matching black/white list entry.
- Email Regular Expression
The FortiGate unit compares the sender email address, as shown in the message envelope MAIL FROM, to the pattern in the patterned field. The regular expression that can be used is much more sophisticated than a simple wildcard variable. If a match is found, the FortiGate unit will take the action configured for the matching black/white list entry.
The pattern field is for entering the identifying information that will enable the filter to correctly identify the email messages.
- If the type is IP/Netmask the filter will be an IP address with a subnet mask.
- If the type is Email Wildcard the filter will be an email address with a wildcard symbol in place of the variable characters. For example *.example.com or fred@*.com.
- If the type is Email Regular Expression, regular expression can be used to create a more granular filter for email addresses. For example, ^[_a-z0-9-]+(\.[_a-z0-9-]+)*@(example|xmple|examp).(com|org|net) could be used filter based on a number of combinations of email domain names.
- Mark as Spam
If this is the selected action, the email will be allowed through but it will be tagged with an indicator that clearly marks the email as spam.
- Mark as Clear
If this is the selected action, the email will be allowed to go through to its destination on the assumption that the message is not spam.
- Mark as Reject
If this is the selected action, the email will be dropped at the before reaching its destination.
Indicates whether this particular list is enabled or disabled
Banned word check
When you enable banned word checking, your FortiGate unit will examine the email message for words appearing in the banned word list specified in the email filter profile. If the total score of the banned word discovered in the email message exceeds the threshold value set in the email filter profile, your FortiGate unit will treat the message as spam.
When determining the banned word score total for an email message, each banned word score is added once no matter how many times the word appears in the message. Use the command
config spamfilter bword to add an email banned word list. Use the command
config spamfilter profile to add a banned word list to an email filtering profile.
How content is evaluated
Every time the banned word filter detects a pattern in an email message, it adds the pattern score to the sum of scores for the message. You set this score when you create a new pattern to block content. The score can be any number from zero to 99999. Higher scores indicate more offensive content. When the total score equals or exceeds the threshold, the email message is considered as spam and treated according to the spam action configured in the email filter profile. The score for each pattern is counted only once, even if that pattern appears many times in the email message. The default score for banned word patterns is 10 and the default threshold is 10. This means that by default, an email message is blocked by a single match.
A pattern can be part of a word, a whole word, or a phrase. Multiple words entered as a pattern are treated as a phrase. The phrase must appear as entered to match. You can also use wildcards or regular expressions to have a pattern match multiple words or phrases.
For example, the FortiGate unit scans an email message that contains only this sentence: “The score for each word or phrase is counted only once, even if that word or phrase appears many times in the email message.”
|Banned word pattern||Pattern type||Assigned score||Score added to the sum for the entire page||Comment|
|word||Wildcard||20||20||The pattern appears twice but multiple occurrences are only counted once.|
|word phrase||Wildcard||20||0||Although each word in the phrase appears in the message, the words do not appear together as they do in the pattern. There are no matches.|
|word*phrase||Wildcard||20||20||The wildcard represents any number of any character. A match occurs as long as “word” appears before “phrase” regardless of what is in between them.|
|mail*age||Wildcard||20||20||Since the wildcard character can represent any characters, this pattern is a match because “email message” appears in the message.|
In this example, the message is treated as spam if the banned word threshold is set to 60 or less.
Adding words to a banned word list
When you enter a word, set the
Pattern-type to wildcards or regular expressions.
Wildcard uses an asterisk (“*”) to match any number of any character. For example, re* will match all words starting with “re”.
Regular expression uses Perl regular expression syntax. See https://perldoc.perl.org/perlretut.html for detailed information about using Perl regular expressions.
DNS-based Blackhole List (DNSBL)
A DNSBL is a list of IP addresses, usually maintained by a third party, which are identified as being associated with spamming.
FortiGuard IP address check
The FortiGate unit queries the FortiGuard Antispam Service to determine if the IP address of the client delivering the email is blacklisted. A match will cause the FortiGate unit to treat delivered messages as spam.
The default setting of the
smtp-spamhdrip CLI command is
disable. When you enable FortiGuard IP address checking, your FortiGate unit will submit the IP address of the client to the FortiGuard service for checking. If the IP address exists in the FortiGuard IP address black list, your FortiGate unit will treat the message as spam.
FortiGuard URL check
When you enable FortiGuard URL checking, your FortiGate unit will submit all URLs appearing in the email message body to the FortiGuard service for checking. If a URL exists in the FortiGuard URL black list, your FortiGate unit will treat the message as spam.
FortiGuard email checksum check
When you enable FortiGuard email checksum checking, your FortiGate unit will submit a checksum of each email message to the FortiGuard service for checking. If a checksum exists in the FortiGuard checksum black list, your FortiGate unit will treat the message as spam.
Detect phishing URLs in email
When you enable FortiGuard phishing URL detection, your FortiGate unit will submit all URL hyperlinks appearing in the email message body to the FortiGuard service for checking. If a URL exists in the FortiGuard URL phishing list, your FortiGate unit will remove the hyperlink from the message. The URL will remain in place, but it will no longer be a selectable hyperlink.
FortiGuard spam submission
Spam submission is a way you can inform the FortiGuard AntiSpam service of non-spam messages incorrectly marked as spam. When you enable this setting, the FortiGate unit adds a link to the end of every message marked as spam. You then select this link to inform the FortiGuard AntiSpam service when a message is incorrectly marked.
Trusted IP Addresses
A list if IP addresses that are trusted by the FortiGate is created. Any email traffic coming in from these IP address will be exempted to perform IP based check, such as DNSBL/RBL, FortiShield SPAM IP or locally defined IP black list check.
If the FortiGate unit sits behind a company’s Mail Transfer Units, it may be unnecessary to check email IP addresses because they are internal and trusted. The only IP addresses that need to be checked are those from outside of the company. In some cases, external IP addresses may be added to the list if it is known that they are not sources of spam.
This feature filters by the MIME header. MIME header settings are configured in a separate part of the command tree but MIME header filtering is enabled within each profile.
HELO DNS lookup
Whenever a client opens an SMTP session with a server, the client sends a HELO command with the client domain name. The FortiGate unit takes the domain name specified by the client in the HELO and does a DNS lookup to determine if the domain exists. If the lookup fails, the FortiGate unit determines that any messages delivered during the SMTP session are spam.
The HELO DNS lookup is available only for SMTP traffic.
Return email DNS check
The FortiGate unit performs a DNS lookup on the If no such record exists, the message is treated as spam.
When you enable return email DNS checking, your FortiGate unit will take the domain in the reply-to email address and reply-to domain and check the DNS servers to see if there is an A or MX record for the domain. If the domain does not exist, your FortiGate unit will treat the message as spam.