私もこれを認識しました。11月28日にDiscourseを更新した後、エラーメッセージJob exception: :enable_starttls and :tls are mutually exclusive. Set :tls if you're on an SMTPS connection. Set :enable_starttls if you're on an SMTP connection and using STARTTLS for secure TLS upgrade.を伴うメール配信エラーが発生し始めました。
バックトレース
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/mail-2.9.0/lib/mail/network/delivery_methods/smtp.rb:159:in `build_smtp_session'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/mail-2.9.0/lib/mail/network/delivery_methods/smtp.rb:154:in `start_smtp_session'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/mail-2.9.0/lib/mail/network/delivery_methods/smtp.rb:108:in `deliver!'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/mail-2.9.0/lib/mail/message.rb:269:in `deliver!'
/usr/local/lib/ruby/3.3.0/delegate.rb:87:in `method_missing'
/var/www/discourse/lib/email/sender.rb:296:in `send'
/var/www/discourse/app/jobs/regular/user_email.rb:80:in `send_user_email'
/var/www/discourse/app/jobs/regular/user_email.rb:40:in `execute'
/var/www/discourse/app/jobs/base.rb:318:in `block (2 levels) in perform'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/rails_multisite-7.0.0/lib/rails_multisite/connection_management/null_instance.rb:49:in `with_connection'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/rails_multisite-7.0.0/lib/rails_multisite/connection_management.rb:17:in `with_connection'
/var/www/discourse/app/jobs/base.rb:305:in `block in perform'
/var/www/discourse/app/jobs/base.rb:301:in `each'
/var/www/discourse/app/jobs/base.rb:301:in `perform'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:220:in `execute_job'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:185:in `block (4 levels) in process'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:180:in `traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:183:in `block in traverse'
/var/www/discourse/lib/sidekiq/suppress_user_email_errors.rb:6:in `call'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:182:in `traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:183:in `block in traverse'
/var/www/discourse/lib/sidekiq/discourse_event.rb:6:in `call'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:182:in `traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:183:in `block in traverse'
/var/www/discourse/lib/sidekiq/pausable.rb:131:in `call'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:182:in `traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:183:in `block in traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/job/interrupt_handler.rb:9:in `call'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:182:in `traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:183:in `block in traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/metrics/tracking.rb:26:in `track'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/metrics/tracking.rb:134:in `call'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:182:in `traverse'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/middleware/chain.rb:173:in `invoke'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:184:in `block (3 levels) in process'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:145:in `block (6 levels) in dispatch'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/job_retry.rb:118:in `local'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:144:in `block (5 levels) in dispatch'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/config.rb:39:in `block in <class:Config>'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:139:in `block (4 levels) in dispatch'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:281:in `stats'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:134:in `block (3 levels) in dispatch'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/job_logger.rb:15:in `call'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:133:in `block (2 levels) in dispatch'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/job_retry.rb:85:in `global'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:132:in `block in dispatch'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/job_logger.rb:40:in `prepare'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:131:in `dispatch'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:183:in `block (2 levels) in process'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:182:in `handle_interrupt'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:182:in `block in process'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:181:in `handle_interrupt'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:181:in `process'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:86:in `process_one'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/processor.rb:76:in `run'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/component.rb:10:in `watchdog'
/var/www/discourse/vendor/bundle/ruby/3.3.0/gems/sidekiq-7.3.9/lib/sidekiq/component.rb:19:in `block in safe_thread'
その間にコミット履歴を確認したところ、問題が私の環境で発生したアップデートのわずか3日前に、mail gemがバージョン2.9.0に更新されていることに気づきました(#36254)。そのリリースノートには、「SMTP: refactor and accept starttls :always and :auto by @eval in #1536」という記述があります。Rubyについてはあまり詳しくありませんが、参照されているプルリクエストのこの変更は私には少し怪しく見えます。