Sending specially crafted email can cause mailbox to have permanently unaccessible mail, or the mail can be stuck in delivery.
External References: https://dovecot.org/pipermail/dovecot-news/2020-February/000430.html
This issue is already addressed in Fedora via the following updates: Fedora-30-testing: http://koji.fedoraproject.org/packages/dovecot/2.3.9.3/1.fc30 Fedora-31-testing: http://koji.fedoraproject.org/packages/dovecot/2.3.9.3/1.fc31
Upstream commit: https://github.com/dovecot/core/commit/3a55f35c208b5fd3d52c0a6272bd5b8717a2ae54