Redmine email sending stops after changing from smtp to async_smtp - redmine

Question

I have setup redmine and configure email sending feature with smtp.
It takes around 15 seconds for any user action to complete if email is enabled( sending email takes time at our smtp server as it has a configured delay).
So I have tried using async_smtp as shown below.
production:
delivery_method: :async_smtp
async_smtp_settings:
enable_starttls_auto: true
address: "smtp.xxx.com"
port: 25
domain: "smtp.xx.com"
authentication: :plain
user_name: "yyy#xxx.com"
password: "xxx!"
Redmine shows that email is sent, but I couldn't see the email. Log also doesn't show any error.
Can someone help?
Adding more detail below:
with the above settings, I get success log as shown below:
Sent email "Redmine test" (16ms) to: [email]
Redirected to http://[ip]/redmine/settings?tab=notifications Completed 302 Found in
328ms (ActiveRecord: 0.0ms) Started GET
"/redmine/settings?tab=notifications" for [ip] at 2015-10-05 15:13:04+0530
note: I have replaced ip and email with [ip] and [email]

Answer 1

I made it work!
I found that you need to add extra level in the config file, email_delivery. Its hinted at elsewhere in the file, but all the examples on redmine.org miss it out. Seems that only async requires it.
production:
email_delivery:
delivery_method: :async_smtp
async_smtp_settings:
address: ...

Answer 2

It's look like it's the same problem I had, your smtp server can't reroute your request. I had to let my configuration to smtp instead of async_smtp as our smtp server can't handle it.
Is it your own smtp server or a hotmail, gmail, etc?
BTW can you try to add this config.action_mailer.logger = nil to your config/environments/production.rb configuration file, and give us the output of log/production.log
EDIT: Look like redmine have problem handling certain SSL certificate.