The module is loaded, when the configuration file contains section [mod_sieve].
It executes a Sieve script on every incoming mail per recipient.
The only actions executed are redirect, reject (rejects at SMTP level), explicit keep and discard. Actions fileinto and vacation are ignored. Cyrus Imap 3.1 has action ereject, but aegee-milter works with 3.0 and 3.0 has no ereject.
Calling the Sieve extensions from RFC 5490 is undefined behaviour.
Long rejection lines are split (folded) by aegee-milter when sent over SMTP, so the Sieve script can contain very long text after the reject action.
The executed script is the default script for the recipient. If there is no such script, then the default global script for the domain is executed. If there is no such script, mod_sieve does nothing.