Sieve interpreter is not protected against abusive scripts that claim excessive resource usage. Especially scripts using massive amounts of regexps. Attacker can DoS the mail delivery system by using excessive amount of CPU and/or reaching the lmtp/lda process limits.


Workaround: Disabling the regex sieve extension avoids the worst problems. lmtp_user_concurrency_limit may also be helpful.

Reference : https://dovecot.org/pipermail/dovecot-news/2021-June/000460.html