Explanation
Sieve filters are usually applied to an email when the email is delivered to a mailbox by a mail delivery agent (MDA) or a local delivery agent (LDA). Sieve filters are scripts that can perform various actions on incoming emails, such as sorting them into folders, assigning labels, forwarding, rejecting, or discarding them. Sieve filters are executed on the server side, and they are independent of the mail user agent (MUA) or the mail access protocol. This means that the email filtering is consistent across different email clients and devices.
Sieve filters are not applied when the email is relayed by an SMTP server, received by an SMTP smarthost, sent to the first server by an MUA, or retrieved by an MUA. These are different stages of the email delivery process, but they do not involve the final delivery of the email to a mailbox.
References:
LPIC-2 Exam 202 Objectives, Objective 205.4: Managing a dovecot server
Sieve filter (advanced custom filters) | Proton
start - Sieve.Info
What is Sieve Filtering? - Knowledge Base - Pair Networks