1. Login to your Gmail account where you want to add your domain's mail account as SMTP.
2. Go to Settings→ "Accounts and Import" tab.
3. Click on "Add another email address" you own in "Send mail as" option.
4. In next window, put name and email account that you want to configure and click on next.
5. In next page, use below settings:
SMTP server: mail.domain.com
Username: Full email account
Password: Password of your email account
6. Select Unsecured connection and click on Add account.
You will receive confirmation code at your domain's email account, just find it and put in GMail's next window.