Filters

Filters are Sieve scripts. Sieve is deliberately not a general-purpose language: no loops, no recursion, no way to call out. That is the reason it is safe to run a customer's script on the delivery path at all.

A filter belongs to the account and can be attached to any number of mailboxes.

Supported#

Control — if / elsif / else, stop, require (accepted and ignored).

Tests — address, envelope, header, exists, size, true, false, not, allof, anyof.

Comparators — :is, :contains, :matches (with * and ?), :regex.

Address parts — :all, :localpart, :domain.

Actions — keep, discard, fileinto (with :create), redirect, reject, addflag, setflag, removeflag.

The implicit keep#

If a script takes no filing action, the message is delivered to the inbox anyway. fileinto suppresses that; an explicit keep alongside it delivers to both.

Examples#

Newsletters out of the inbox:

require ["fileinto"];

if anyof (exists "list-id", header :contains "precedence" "bulk") {
  fileinto :create "Newsletters";
  stop;
}

Flag anything from your own domain:

require ["imap4flags"];

if address :domain :is "from" "example.com" {
  addflag "\\Flagged";
}

File by sub-address:

require ["fileinto", "envelope"];

if envelope :localpart :matches "to" "*+receipts" {
  fileinto :create "Receipts";
}

Drop something silently — no bounce, so the sender learns nothing:

if header :contains "subject" "unmissable opportunity" {
  discard;
}

Notes#

• A script that fails to compile is rejected on save.
• A script that throws at delivery time is treated as absent — the message is delivered to the inbox and the error is shown against the filter. A broken filter never loses mail.
• reject refuses the message with a 550. discard accepts and drops it. Rejecting tells the sender their message was filtered; discarding does not.
• Scripts stop after 100 actions.