Please find in this article the steps to set up the SMTP service for gmail accounts.
1. Turn on 2 step verification of your Google account (if not already turned on)
2. Create an App Password for erplain.app
Instructions from Google: https://support.google.com/mail/answer/185833
3. Select 'Other (Custom name)'
4. Add 'erplain.app"
5. Open erplain and complete set up
Once the password is generated, go to Settings > Users > Choose user > Edit.
Don't forget to save and send a test email to make sure the configuration is done.