redmine四种邮件配置方式和优化redmine服务器速度

delivery_method有效设置为以下四种：

:smtp

:async_smtp

:sendmail

:async_sendmail

production:
email_delivery:
delivery_method: :smtp
smtp_settings:
address: smtp.example.net
port: 25
domain: example.net
authentication: :login
user_name: redmine@example.net
password: redmine

production:
email_delivery:
delivery_method: :smtp
smtp_settings:
enable_starttls_auto: true
address: "smtp.gmail.com"
port: '587'
domain: "smtp.gmail.com"
authentication: :plain
user_name: "your_email@gmail.com"
password: "your_password"

production:
email_delivery:
delivery_method: :smtp
smtp_settings:
address: smtp.knology.net
port: 25
domain: cybersprocket.com
authentication: :none

production:
email_delivery:
delivery_method: :sendmail