docker-mailserver/docs/content/advanced/mail-sieve.md

User-defined sieve filters

Sieve allows to specify filtering rules for incoming emails that allow for example sorting mails into different folders depending on the title of an email.

To specify a user-defined Sieve filter place a .dovecot.sieve file into a virtual user's mail folder e.g. /var/mail/domain.com/user1/.dovecot.sieve. If this file exists dovecot will apply the filtering rules.

An example of a sieve filter that moves mails to a folder INBOX/spam depending on the sender address:

require ["fileinto", "reject"];

if address :contains ["From"] "spam@spam.com" {
   fileinto "INBOX.spam";
} else {
     keep;
}

Note: that folders have to exist beforehand if sieve should move them.

For more examples or a detailed description of the Sieve language have a look at the official site.