Redmine: E-Mail error since 2.x

Since the upgrade to Redmine 2.x, I got problems when Redmine should send e-mails to our Exchange 2010 server. When I want to send the test e-mail (from settings, email notifications), the page shows me this error:

An error occurred while sending mail (SSL_connect returned=1 errno=0 state=SSLv2/v3 read server hello A: unknown protocol)

This is the relevant part of my configuration.yml:

# default configuration options for all environments
default:
  # Outgoing emails configuration (see examples above)
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      address: our-server
      port: 25
      #enable_starttls_auto: true
      openssl_verify_mode: none
      #domain: example.net
      #authentication: :login
      #user_name: 
      #password:

This worked fine with Redmine 1.x, but since I upgraded from Redmine 1.3.1 to 2.2.3 and last week to 2.3.1, e-mail notifications seems to be mostly gone (some users said they got e-mails some weeks ago, but I cannot confirm this).

Of course I also looked in my production.log and I even raised the log level to debug, but there are no errors in this file.

What can I further do to solve this problem?