twisted is used in matrix-synapse for smtp handling.
Mostly this is used for password resets, but also notifications
are delivered that way.
older versions of twisted require the e-mail server to
have TLS1.0 enabled.
Obviously, quite a lot of servers have this disabled which means
synapse won't be able to deliver mails using such servers.
matrix-synapse issue:
https://github.com/matrix-org/synapse/issues/6211
Maximilian Güntner