This is a config option ported over from DINUM's Sydent: https://github.com/matrix-org/sydent/pull/285
They've switched to validating 3PIDs via Synapse rather than Sydent, and would like to retain this functionality.
This original purpose for this change is phishing prevention. This solution could also potentially be replaced by a similar one to https://github.com/matrix-org/synapse/pull/8004, but across all `*/submit_token` endpoint.
This option may still be useful to enterprise even with that safeguard in place though, if they want to be absolutely sure that their employees don't follow links to other domains.
* commit 'f88c48f3b':
1.18.0rc1
Fix error reporting when using `opentracing.trace` (#7961)
Fix typing replication not being handled on master (#7959)
Remove hacky error handling for inlineDeferreds. (#7950)
Convert tests/rest/admin/test_room.py to unix file endings (#7953)
Support oEmbed for media previews. (#7920)
Convert state resolution to async/await (#7942)
Fix up types and comments that refer to Deferreds. (#7945)
Do not convert async functions to Deferreds in the interactive_auth_handler (#7944)
Convert more of the media code to async/await (#7873)
Return an empty body for OPTIONS requests. (#7886)
Downgrade warning on client disconnect to INFO (#7928)
Convert presence handler helpers to async/await. (#7939)
Update the auth providers to be async. (#7935)
Put a cache on `/state_ids` (#7931)
* commit 'de119063f': (31 commits)
Convert room list handler to async/await. (#7912)
Element CSS and logo in email templates (#7919)
Lint the contrib/ directory in CI and linting scripts, add synctl to linting script (#7914)
Remove unused code from synapse.logging.utils. (#7897)
Fix a typo in the sample config. (#7890)
Fix deprecation warning: import ABC from collections.abc (#7892)
Change sample config's postgres user to synapse_user (#7889)
Fix deprecation warning due to invalid escape sequences (#7895)
Remove Ubuntu Eoan that is now EOL (#7888)
Fix the trace function for async functions. (#7872)
Add help for creating a user via docker (#7885)
Switch to Debian:Slim from Alpine for the docker image (#7839)
Stop using 'device_max_stream_id' (#7882)
Fix TypeError in synapse.notifier (#7880)
Add a default limit (of 100) to get/sync operations. (#7858)
Change "unknown room ver" logging to warning. (#7881)
Convert device handler to async/await (#7871)
Convert synapse.app to async/await. (#7868)
Convert _base, profile, and _receipts handlers to async/await (#7860)
Add admin endpoint to get members in a room. (#7842)
...
* commit 'a973bcb8a':
Add some tiny type annotations (#7870)
Remove obsolete comment.
Ensure that calls to `json.dumps` are compatible with the standard library json. (#7836)
Avoid brand new rooms in `delete_old_current_state_events` (#7854)
Allow accounts to be re-activated from the admin APIs. (#7847)
Fix tests
Fix typo
Newsfile
Use get_users_in_room rather than state handler in typing for speed
Fix client reader sharding tests (#7853)
Convert E2E key and room key handlers to async/await. (#7851)
Return the proper 403 Forbidden error during errors with JWT logins. (#7844)
remove `retry_on_integrity_error` wrapper for persist_events (#7848)
* commit '43726783e': (22 commits)
1.17.0rc1
Fix some spelling mistakes / typos. (#7811)
`update_membership` declaration: now always returns an event id. (#7809)
Improve stacktraces from exceptions in background processes (#7808)
Fix `can only concatenate list (not "tuple") to list` exception (#7810)
Pass original request headers from workers to the main process. (#7797)
Generate real events when we reject invites (#7804)
Add `HomeServer.signing_key` property (#7805)
Revert "Update the installation docs on apt-transport-https (#7801)"
Do not use simplejson in Synapse. (#7800)
Stop passing bytes when dumping JSON (#7799)
Update the installation docs on apt-transport-https (#7801)
shuffle changelog slightly
Change Caddy links (old is deprecated) (#7789)
Stop populating unused table `local_invites`. (#7793)
Refactor getting replication updates from database v2. (#7740)
Add libwebp dependency to Dockerfile (#7791)
Add documentation for JWT login type and improve sample config. (#7776)
Convert the appservice handler to async/await. (#7775)
Don't ignore `set_tweak` actions with no explicit `value`. (#7766)
...
