common/session.c in Claws Mail before 3.17.6 has a protocol violation because suffix data after STARTTLS is mishandled.
lists.opensuse.org/opensuse-security-announce/2020-07/msg00090.html
lists.opensuse.org/opensuse-security-announce/2020-08/msg00002.html
lists.opensuse.org/opensuse-security-announce/2020-08/msg00060.html
lists.opensuse.org/opensuse-security-announce/2020-09/msg00051.html
lists.opensuse.org/opensuse-security-announce/2020-11/msg00013.html
git.claws-mail.org/?p=claws.git%3Ba=blob%3Bf=RELEASE_NOTES
git.claws-mail.org/?p=claws.git%3Ba=commit%3Bh=fcc25329049b6f9bd8d890f1197ed61eb12e14d5
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/6YVQB7NRBHO67Q74RS7RZCMW4ENRVBB4/
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/G7UX65342HRVDQML4G4GEVEUB764EUM5/
security.gentoo.org/glsa/202007-56