In Dovecot before 2.3.11.3, uncontrolled recursion in submission, lmtp, and lda allows remote attackers to cause a denial of service (resource consumption) via a crafted e-mail message with deeply nested MIME parts.
https://www.debian.org/security/2020/dsa-4745
https://usn.ubuntu.com/4456-2/
https://usn.ubuntu.com/4456-1/
https://security.gentoo.org/glsa/202009-02
https://lists.debian.org/debian-lts-announce/2020/08/msg00024.html