Redmine

The (default) ActiveJob::QueueAdapters::AsyncAdapter for delayed jobs, including sending of mails, works by creating a separate thread in your application server processes in order to process the jobs. It may be that some of the mail sending jobs fail, e.g. because the user's email address is invalid. Depending on your application server and its available resources and configured process handling policies, the jobs may also be lost of the application server process exits before all jobs are processed.

You should check the logs in logs/production.log on your server for any errors. For production use, you should also move to a different queuing implementation such as sidekiq which support persistent jobs.

Please try to find the cause of this issue or at least a define reproducer for this issue based on your setup. Unfortunately, without more details which would allow us to reproduce this issue, it is hard to be of any actual help.

Thank you for your advice.
According to the production.log file, there was an error called "Email delivery error: end of file reached".
As a result, although there are 35 users in the group, only 10 users were able to send email.

Succeed User-ID: 5,6,7,8,10,11,13,19,20 and 21
Failed User-ID: Others

- configuration.yml
production:
email_delivery:
delivery_method: :smtp
smtp_settings:
enable_starttls_auto: true
address: "Server name"
port: 587
authentication: :login
domain: 'Domain name'
user_name: 'Email address'
password: 'Password'

Since there are users who can send email, I don't think there is anything wrong with the email environment, but what do you think?
I attached log file a part of sending email just in case.