SSL_connect returned=1 errno=0 state=unknown state: unknown protocol

After reading through many posts I couldn't find a solution to my problem. I've made a test connection with www.smtper.net which was successful. My error.log only tells me the error:

OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=unknown state: unknown protocol

Maybe it has something to do with my nginx settings?

Issue

I'm trying to configure my smtp via mailgun (or gmail) but without success. My configuration in developement.rb looks like:

config.action_mailer.default_url_options = { :host => "xxxxx:8765" } 
config.action_mailer.delivery_method = :smtp
config.action_mailer.perform_deliveries = true
config.action_mailer.smtp_settings = {
   :address => "smtp.gmail.com",
   :port => 587,
   :domain => "mail.google.com",
   :user_name => "xxx@gmail.com",
   :password => "xxxxxx",
   :authentication => :plain,
   :enable_starttls_auto => true
}

I have installed OpenSSL 1.0.1f on my development server