util/emailutils.py in Matrix Sydent before 1.0.2 mishandles registration restrictions that are based on e-mail domain, if the allowed_local_3pids option is enabled. This occurs because of potentially unwanted behavior in Python, in which an email.utils.parseaddr call on [email protected]@good.example.com returns the [email protected] substring.
https://twitter.com/matrixdotorg/status/1118934335963500545
https://matrix.org/blog/2019/04/18/security-update-sydent-1-0-2/
https://github.com/matrix-org/sydent/compare/7c002cd...09278fb
https://github.com/matrix-org/sydent/commit/4e1cfff53429c49c87d5c457a18ed435520044fc