Since 2018-11-13, https://discourse-mediawiki.wmflabs.org/ and discourse.wmflabs.org are sending emails only to @wikimedia.org addresses. Anything sent to any other domain is skipped with "550 Relay not permitted".
Can this be related to T208830? Or has anything changed in mx1001.wikimedia.org?
These are the current settings:
Delivery Method address mx1001.wikimedia.org port 25 authentication plain enable_starttls_auto true