diff --git a/deploy/forum/app.yml b/deploy/forum/app.yml
index aa91299..be1393f 100644
--- a/deploy/forum/app.yml
+++ b/deploy/forum/app.yml
@@ -44,6 +44,9 @@ env:
   DISCOURSE_SMTP_USER_NAME: '{{FORUM_SMTP_USER}}'
   DISCOURSE_SMTP_PASSWORD: '{{FORUM_SMTP_PASSWORD}}'
   DISCOURSE_SMTP_ENABLE_START_TLS: true
+  ## Discourse defaults to AUTH PLAIN; Microsoft 365 (smtp.office365.com) only offers LOGIN and XOAUTH2
+  ## and answers PLAIN with "504 5.7.4 Unrecognized authentication type". LOGIN works on other servers too.
+  DISCOURSE_SMTP_AUTHENTICATION: login
   DISCOURSE_NOTIFICATION_EMAIL: '{{FORUM_NOTIFICATION_EMAIL}}'
 
 volumes: