The production.log when the error occurs is as follows

Processing by QuestionsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"10KbTWH9h0qFix4wy/MzNh399mRhwrrJBoUPlzY/XJUlX2RZmJ3r9fwnOTZz9UJKvd5JvWpV0/L9Hs2RGHg2qw==", "question"=>{"subject"=>"退職手続きのプロセス及び担当者", "locked"=>"0", "section_id"=>"4", "content"=>"テスト", "tag_list"=>["", ""]}, "commit"=>"作成", "project_id"=>"3002402"}
Current user: 0007 (id=5)
Rendering plugins/redmine_questions/app/views/mailer/question_question_added.text.erb within layouts/mailer
Rendered plugins/redmine_questions/app/views/mailer/question_question_added.text.erb within layouts/mailer (0.5ms)
Rendering plugins/redmine_questions/app/views/mailer/question_question_added.html.erb within layouts/mailer
Rendered plugins/redmine_questions/app/views/mailer/question_question_added.html.erb within layouts/mailer (2.0ms)
Completed 500 Internal Server Error in 2408ms (ActiveRecord: 128.1ms)

Net::SMTPAuthenticationError (530-5.7.0 Authentication Required. Learn more at
):

Environment used
Environment:
Redmine version 4.1.0.stable
Ruby version 2.5.7-p206 (2019-10-01) [x64-mingw32]
Rails version 5.2.4.1
Environment production
Database adapter Mysql2
Mailer queue ActiveJob::QueueAdapters::AsyncAdapter
Mailer delivery smtp
SCM:
Git 2.25.0
Filesystem