Python Milter Mail PolicyThe
These are the policies implemented by the
Classify connectionWhen the SMTP client connects, the connection IP address is saved for later verification, and the connection is classified as INTERNAL or EXTERNAL by matching the ip address against the
Examples from the log file (not the SMTP error message returned):
2005Jul29 13:56:53  connect from p50863492.dip0.t-ipconnect.de at ('18.104.22.168', 1858) EXTERNAL DYN 2005Jul29 18:10:15  connect from foopub at ('22.214.171.124', 46513) EXTERNAL TRUSTED 2005Jul29 14:41:00  connect from foobar at ('192.168.0.1', 41205) INTERNAL 2005Jul29 14:41:15  connect from cncln.online.ln.cn at ('126.96.36.199', 35992) EXTERNAL
Certain obviously evil PTR names are blocked at this point: "localhost" (when IP is not 127.*) and ".".
2005Jul29 14:49:50  connect from localhost at ('188.8.131.52', 50507) EXTERNAL 2005Jul29 14:49:50  REJECT: PTR is localhost
HELO CheckThe HELO name provided by the client is saved for later verification (for example by SPF). We could validate the HELO at this point by verifying that an A record for the HELO name matches the connect ip. However, currently we only block certain obvious problems. HELO names that look like an IP4 address and ones that match the
2005Jul29 18:10:15  hello from example.com 2005Jul29 18:10:15  REJECT: spam from self: example.com 2005Jul29 18:17:09  hello from 184.108.40.206 2005Jul29 18:17:09  REJECT: numeric hello name: 220.127.116.11
MAIL FROM CheckBefore calling our milter, sendmail checks a DNS blacklist to block banned sender domains. We never see a blocked domain.
The MAIL FROM address is saved for possible use by the smart-alias
feature. First, the
wiretapThe wiretap feature can screen and/or monitor mail to/from certain users. If the MAIL FROM is being wiretapped, the recipients are altered accordingly.
SPF checkThe MAIL FROM, connect IP, and HELO name are checked against any SPF records published via DNS for the alleged sender (MAIL FROM) to determine the official SPF policy result. The offical SPF result is then logged in the Received-SPF header field, but certain results are subjected to further processing to create an effective result for policy purposes.
If the official result is 'none', we try to turn it into an effective result of
'pass' or 'fail'. First, we check for a local substitute SPF record
under the domain defined in the
If there is no local SPF record, and the effective result is still not
'pass', we check for either a valid HELO name or a valid PTR record for
the connect IP. A valid HELO or PTR cannot look like a dynamic name
as determined by the heuristic in
If HELO has an SPF record, and the result is anything but pass, we reject the connection:
2005Jul30 19:45:16  connect from [18.104.22.168] at ('22.214.171.124', 3581) EXTERNAL DYN 2005Jul30 19:45:18  hello from adelphia.net 2005Jul30 19:45:19  mail from <email@example.com> () 2005Jul30 19:45:19  REJECT: hello SPF: fail 550 access deniedNote that HELO does not have any forwarding issues like MAIL FROM, and so any result other than 'pass' or 'none' should be treated like 'fail'.
Only if nothing about the SMTP envelope can be validated does the effective result remain 'none. I call this the "3 strikes" rule.
If the official result is 'permerror' (a syntax error in the sender's policy), we use the 'lax' option in pyspf to try various heuristics to guess what they really meant. For instance, the invalid mechanism "ip:126.96.36.199" is treated as "ip4:188.8.131.52". The result of lax processing is then used as the effective result for policy purposes.
With an effective SPF result in hand, we consult the sendmail access database to find our receiver policy for the sender.
SPF policy syntaxFirst, the full sender is checked:
SPF-Fail:firstname.lastname@example.org DSNThis says to accept mail from that adelphia.net user despite the SPF fail, but only after annoying them with a DSN about their ISP's broken policy.
If there is no match on the full sender, the domain is checked:
SPF-Neutral:aol.com REJECTThis says to reject mail from AOL with an SPF result of neutral. This means AOL users can't use their AOL address with another mail service to send us mail. This is good because the other mail service is likely a badly configured greeting card site or a virus.
Finally, a default policy for the result is checked. While there are program defaults, you should have defaults in the access database for SPF results:
SPF-Neutral: CBV SPF-Softfail: DSN SPF-PermError: DSN SPF-TempError: REJECT SPF-None: REJECT SPF-Fail: REJECT SPF-Pass: OK
ReputationIf the sender has not been rejected by this point, and if a GOSSiP server is configured, we consult GOSSiP for the reputation score of the sender and SPF result. The score is a number from -100 to 100 with a confidence percentage from 0 to 100. A really bad reputation (less than -50 with confidence greater than 3) is rejected. Note that the reputation is tracked independently for each SPF result and sender combination. So aol.com:neutral might have a really bad reputation, while aol.com:pass would be ok. Furthermore, when a sender finally publishes an SPF policy and starts getting SPF pass, their reputation is effectively reset.
Whitelists and BlacklistsThe administrator can whitelist or blacklist senders and sending domains by appending them to
Real Soon Now users will be able to maintain their own whitelist and blacklist that applies only when they are the recipient.
Recipient CheckWhen the pysrs package is installed and configured, outgoing mail is "signed" by adding a cryto-cookie to MAIL FROM. All DSNs (null MAIL FROM) must be sent to a MAIL FROM address only, so a DSN without a validated cookie in RCPT is immediately rejected. Forwarded domains can have a list of valid recipients configured, and invalid recipients are rejected. The MTA rejects invalid local RCPTs. Four or more invalid RCPTs cause the IP to be blacklisted.
Content FilterMost messages have been rejected or delivered by now, but spammers are always finding new places to send their junk from. For instance, we get around 10000 emails a day, of which around 500 are first time spam senders. A bayesian filter is trained by the whitelists and blacklists, and scores the message. What is likely spam is either rejected or quarantined. If the sender is an effective SPF pass, then they get a DSN notifying them that their message has been quarantined. (A DSN failure gets the sender auto blacklisted.) Else, if the reject_spam option is set, the message is rejected. Otherwise, a CBV is done (failure gets the sender auto blacklisted) and the message is silently quarantined.
Normally, you don't want email messages to silently disappear into a black hole, so you should set the reject_spam option. However, if you don't want your correspondent's email to get rejected, you can check your quarantine frequently instead.
HoneypotYou can also blacklist recipients by listing them as aliases of the 'honeypot' dspam user. These are collectively called the honeypot. Any email to these recipients is used to train the spam filter as spam and chalk up a reputation demerit for the sender, then discarded. It might be a good idea to blacklist the sender if it has SPF pass as well, but I'm afraid of accidents.
ReputationReputation is tracked by sending domain and effective SPF result. The GOSSiP server tracks the spam/ham status of the last 1024 messages for each domain:result combination. When the server is queried during the SMTP envelope phase (MAIL FROM), it also queries any configured peers, and the scores are combined. Domains with a history of spam for a given SPF result are rejected at MAIL FROM. The GOSSiP system has a command line utility to reset (delete) a reputation for cases where a sender that was infected with malware is repaired. In addition, the confidence score of a reputation decays with time, so a bad sender will eventually be able to try again without manual intervention.