How to set up outgoing mail settings and send an answer to a customer's ticket?

Specify your mailing service and credentials

Update redmine/config/configuration.yml file according to your mailing service:

Local sendmail server:

default:
  email_delivery:
    delivery_method: :sendmail

SMTP server without authorization:

default:
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      address: "smtp.example.net" 
      port: 25
      domain: "<yourdomain>" 

SMTP with login authentication:

default:
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      address: "smtp.example.net" 
      port: 25
      domain: "yourdomain" 
      authentication: :login
      user_name: "yourusername" 
      password: "yourpassword" 

SMTP with plain authentication:

default:
 email_delivery:
   delivery_method: :smtp
   smtp_settings:
     address: "smtp.example.net" 
     port: 25
     domain: "yourdomain" 
     authentication: :plain
     user_name: "yourusername" 
     password: "yourpassword" 

SMTP Gmail with TLS:

default:
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      enable_starttls_auto: true
      address: "smtp.gmail.com" 
      port: 465
      domain: "smtp.gmail.com" 
      authentication: :plain
      user_name: "yourusername@gmail.com" 
      password: "yourpassword" 

SMTP Office 365 with TLS:

default:
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      enable_starttls_auto: true
      address: "smtp.office365.com" 
      port: 587
      domain: "yourdomain" 
      authentication: :login
      user_name: "yourmail@somedomain.smth" 
      password: "yourpassword" 

SMTP Outlook with TLS:

default:
  email_delivery:
    delivery_method: :smtp
    smtp_settings:
      enable_starttls_auto: true
      address: "youraddress.outlook.com" 
      port: 587
      domain: "outlook.com" 
      authentication: :login
      user_name: "yourmail@somedomain.smth" 
      password: "yourpassword" 

Note: Some providers have additional security measures that must be addressed. For example, as of July 15, 2014, Google increased its security measures and now blocks attempts from apps it deems less secure. You can change your Gmail settings here to allow the attempts. If your Gmail account has 2-factor authentication enabled, then you will need to set an app password and use that instead of your regular password.

Reboot Redmine

Set the new email address to be used to send Notifications

Go to Administration->Settings->Email Notifications tab and enter the same email address as used in configuration.yml.

Note: You can specify what name the recipient of the mail sees by using the syntax yourname <yourmail@somedomain.smth>

If the configuration in configuration.yml is missing, you will see the following message:

Test the configuration

At the bottom right of the page, you can find a Send a test email link. By clicking it an email will be sent to the email associated with your account.