1. Navigate to Settings→ Users→ Select a User from the list.

2. Select Actions→ Mail Tracking Settings.

3. Select Type IMAP 

• Set the ‘Type’ to IMAP v2.
• Set the ‘Authentication Method’ to Gmail Service Account OAuth.
• Set the ‘Host Address’ to: imap.gmail.com
• Set the ‘mailbox username’ to the users mailbox address.
• Set the Gmail Service Account address to the service account address that was setup earlier.
• Upload the certificate that was generated earlier (p12) file.
• Add the Secret that was stored in a password manager.
• Tick the ‘Use SSL’ checkbox.

4. SMTP Settings

• Server: smtp.gmail.com
• Authentication method: Gmail Service (OAuth)
• Tick the ‘SMTP SSL’ checkbox.
• Port: 465.

5. Select Test to test the settings.

6. Select Save to save the settings.