libSieve provides a library to interpret Sieve scripts, and to execute those scripts over a given set of messages. The return codes from the libSieve functions let your program know how to handle the message, and then it's up to you to make it so. libSieve makes no attempt to have knowledge of how SMTP, IMAP, or anything else work; just how to parse and deal with a buffer full of emails. The rest is up to you!