1. From the Admin console Home page, go to Appsand thenG Suiteand thenGmailand thenAdvanced settings.
  2. Scroll to the SMTP relay service setting in the Routing section, hover over the setting, and click Configure.
  3. Enter a unique description.
  4. In the Allowed senders section, select the users who are allowed to send messages through the SMTP relay service:
    • Only registered Apps users in my domain—The sender must be a registered user in one of your domains.
    • Only addresses in my domains—The sender doesn't have to be a recognized G Suite user, but must be in one of your registered domains. T
    • Any addresses (not recommended)—The sender address can be anything, even an address outside of your domain.
  5. In the Authentication section, check one or both boxes to set an authentication method:
    • Only accept mail from the specified IP addresses—The system only accepts mail sent from these IP addresses as coming from your domains.
    • Require SMTP Authentication—Enforces the use of SMTP authentication to identify the sending domain. Using this option requires your clients to connect via TLS.
  6. If you chose to only accept messages from specified IP addresses, enter the IP addresses:
    1. Click Add IP RANGE.
    2. Enter a description for the IP address or range.
    3. Enter the IP address or range.
    4. Check the Enabled box to enable (or uncheck to disable) this IP address or range.
    5. Click Save.
  7. Check the Require TLS encryption box 

    Note: If your email server does not support TLS, do not check this box. If you check this box, Google rejects messages that are not encrypted.


  8. Click Add setting or Save.
  9. At the bottom, click Save.
  10. Use the following setting to relay emails:

 

Hostname
smtp-relay.gmail.com.
Port
587 TLS required


              

Alternatively :


Hostname
smtp-relay.gmail.com
Port
25, 587 or 465 (TLS not required)
NOTE: Without TLS encryption, you will be unable to use SMTP authentication and must use IP address authentication.