Redmine

What's your email configuration looks like?

Do you have lines in your production.log telling that the application is parsing mailer views (a.k.a. mailer/*anything*)?

Yes, I have the following lines:

Rendered mailer/_issue.text.erb (3.3ms)
Rendered mailer/issue_edit.text.erb within layouts/mailer (5.3ms)
Rendered mailer/_issue.html.erb (1.2ms)
Rendered mailer/issue_edit.html.erb within layouts/mailer (2.7ms)

Does it work when you change all async_smtp_ to smtp_ (i.e., use synchronous notifications) in your configuration?

Previously it was smtp_.
Tried, did not help.

Does test mail work?

Okay, it is not a bug, it is a misconfiguration.
Some default ActionMailer config may changed.

I had to set enable_starttls_auto to false since there was a hostname does not match CN problem.

On Settings -> Email notifications page after I tried to send the test email.

There was something else instead of CN!

Krisztian Kocsis wrote:

There was something else instead of CN!

It happened in my 2.0.4 install too, and the error is

An error occurred while sending mail (hostname was not match with the server certificate)

Yes, my email server has configuration problems, but worked well with Redmine 1.4.4. I needed include enable_starttls_auto: false in configuration to work again.
My configuration.yml now is likely

production:
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      address: mail.server
      port: 25
      tls: false
      enable_starttls_auto: false
      domain: server.domain
      authentication: :none