Capturing headers and bodies

Out of the box Mailinois only sees log lines: addresses, queue IDs, Message-IDs and SMTP responses. To see headers or bodies, Postfix has to hand it a copy of each message, which it does through a milter. Captures are off until you turn them on.

Modes

capture.mode	What is kept
off	Nothing. The default.
headers	The message’s headers.
full	Headers and body.

capture.max_size caps the bytes kept per message (default 1048576, 1 MiB). Anything past it is cut, and the capture is marked as cut.

Turn it on in Mailinois

Give the milter an address and pick a mode:

listen:
  milter: ":8892"
capture:
  mode: full
  max_size: 1048576

MAILINOIS_MILTER=:8892 does the same as listen.milter. A capture mode needs a milter address: Mailinois will not start with one and not the other.

Point Postfix at it

Add Mailinois to main.cf, after the milters you already have:

/etc/postfix/main.cf

smtpd_milters = <existing milters>, { inet:mailinois:8892, default_action=accept }
non_smtpd_milters = $smtpd_milters
milter_protocol = 6
milter_command_timeout = 5s
milter_content_timeout = 15s

With OpenDKIM on port 8891, for example, the first line reads smtpd_milters = inet:localhost:8891, { inet:mailinois:8892, default_action=accept }. Then reload Postfix with postfix reload.

What each line does:

• smtpd_milters sends mail that arrives over SMTP through the milters in order. List Mailinois after OpenDKIM, so it sees the DKIM-Signature header OpenDKIM adds and can verify it.
• default_action=accept, inside the braces, means that if Mailinois is down or does not answer, Postfix accepts the mail anyway. Mail keeps flowing. That message just has no capture. The braces form needs Postfix 3.0 or later (MILTER_README).
• non_smtpd_milters = $smtpd_milters does the same for mail submitted locally, with sendmail or through the pickup directory.
• milter_protocol = 6 sets the milter protocol version.

Not the global milter_default_action

Set default_action on Mailinois’s milter, not milter_default_action for all of them. Postfix’s default for every milter is tempfail: with only the global changed, a down Rspamd lets spam through and a down OpenDKIM sends mail unsigned, and with it left alone, a plain inet:mailinois:8892 makes Postfix refuse mail while Mailinois is away (4.7.1 Service unavailable, which we saw happen in docker-mailserver). Some images set the global for you: boky/postfix sets it to accept only when its DKIM support is on. The braces form doesn’t depend on any of that.

• milter_command_timeout = 5s and milter_content_timeout = 15s are lower than Postfix’s defaults (30s and 300s), so a stuck Mailinois costs each message seconds, not minutes.

Keep the milter port private. Only Postfix should be able to reach it.

With the agent

When Postfix runs on another network, do not point its milter across the internet: every SMTP transaction would wait on that round trip. Run the milter in the agent on the mail server instead:

mailinois agent -server https://mailinois.example.com -token <ingest token> \
  -file /var/log/mail.log -milter 127.0.0.1:8892 -capture full

Use inet:127.0.0.1:8892 in main.cf. The agent takes the copy locally and sends it on to Mailinois over HTTPS.

What the dashboard shows

A message with a capture shows:

• the parsed headers;
• the text part;
• the HTML part, in a sandboxed frame with scripts and remote content blocked, so opening it loads nothing from the sender;
• attachments, listed by name, type and size;
• DKIM verification of the captured message;
• a download of the .eml, for signed-in users.

has:capture in search finds messages that have one, and GET /api/v1/messages/{id}/eml returns the .eml through the API.

Retention

Captures have their own retention, retention.captures, which defaults to 168h (seven days) against 720h for messages. When a capture expires, the message’s log record stays until retention.messages removes it.

retention:
  messages: 720h
  captures: 168h