1. Install and activate plugin
post-smtp on plugin page on WordPress dashboard, then install and activate.
2-1. Click [Post SMTP] on sidebar menu, then click [Start the Wizard] button
2-2. Sender details
Input your email address to [Email Address] and your name to [Name], then
Click [Next] button
2-3. Outgoing Mail Server Hostname
Input your SMTP server to [Outgoing Mail Server Hostname].
smtp.gmail.com will be set if you set Gmail address in previous step.
2-4. Connectivity Test
Choose SMTP port number and authentication.
If you use Gmail, select [SMTP - gmai.com:587] , then click [Next].
Click [ Google Developers Console Gmail Wizard] to get Client ID and Client Secret.
2-6. Registering application
Choose [Create a project] then click [Continue] button.
2-7. Enabling API
2-8. Add credentials to your project
Select [Gmail API] for [Which API are you using], [Web server] for [When will you be calling the API from?] and [User data] for [What data will you be accessing?] , then click [What credentials do I need?].
2-9. Create an OAuth 2.0 client ID
Then click [Create client ID].
Set same value to [Email address] you've set in step 2-3.
Input plugin name into [Product name shown to users] to easy to find.
Click [I'll do this later].
2-13. Click pencil icon
Make sure to copy [Client ID] and [Client secret], and never tell them others.
Click [Save] after copying it.
2-15.You'll back to setup wizard.
Paste [Client ID] and [Client Secret], then click [Next]
That's all for setting up
3. Send test mail
Click [Send a Test Email] to check mail will be sent to set mail address.