The purpose of this issue is not to implement any feature but to add a notice regarding the removal of a feature done by #26791 to the changelog.

:async_smtp and :async_sendmail delivery methods, which were used for sending emails asynchronously, have been removed by #26791. Redmine 4.0.0 sends emails asynchronously by default using ActiveJob. Therefore, those two delivery methods are no longer necessary.

If you are using: async_smtp or :async_sendmail delivery method on Redmine prior to 4.0.0, you have to update email settings during an upgrading process. Please change config/configuration.yml to use :smtp or :sendmail delivery method instead. Otherwise, Redmine fails to start and log an error message like the following.

Redmine now uses ActiveJob to send emails asynchronously and the :async_smtp delivery method is no longer available.
Please update your config/configuration.yml to use :smtp delivery method instead.