1. Assuming you WorkMail is correctly working by web interface you need to use SES.

   • From SES panel control select "SMTP Settings"
     → "Create My SMTP Credentials"
     → A window about creating user will show up, select 'Create'.
   • Select Show User SMTP Security Credentials, it will show you the SMTP Username and SMTP Password.
     
     

2. From Gmail

   • Setting → Accounts and Import → Add another email address
   • A new yellow window will appear
     name: (the name you wish to display)
     email address: ([email protected])
     Alias checked!
     
     

3. In the next page:

   • smtp server smtp.mail.(your-area).awsapps.com [ex: us-east-1]
     smtp username: ([email protected])
     smtp password: (your Workmail password)
     port: 465
     SSL checked.

A confirm mail will be send to ([email protected]), you can easily access by WorkMail web interface. Click the confirmation link and you will be finally able to send mail as ([email protected]) from Gmail.

In the end, if you wish to receive/redirect on your Gmail every mail sent to WorkMail, you need to create an action in WorkMail's Settings.

• Settings → Email rules → New → Add a Rule name
  → When the message is sent to ... → Select recipient(s) ([email protected])
  → Redirect the message to ... → Select recipient(s) ([email protected])

※ Note: Email rules won't work for recipients in Bcc.

In this way you don't need POP3's setting. I hope this will help some people.