In Dovecot before 2.3.10.1, remote unauthenticated attackers can crash the lmtp or submission process by sending mail with an empty localpart.
https://www.openwall.com/lists/oss-security/2020/05/18/1
https://www.debian.org/security/2020/dsa-4690
https://usn.ubuntu.com/4361-1/
http://www.openwall.com/lists/oss-security/2020/05/18/1
http://seclists.org/fulldisclosure/2020/May/37
http://lists.opensuse.org/opensuse-security-announce/2020-05/msg00059.html