Gmail lets users improve email deliverability and reduce the chance of getting into spam.
When you connect an email account using Gmail Single Click, all emails from this account are sent using Gmail API instead of SMTP. This option is enabled by default. To switch between Gmail API and SMTP:
1. On the Settings page, navigate to the Email Accounts tab and click the needed email account (Gmail Single Click).
2. Expand the Sending and receiving section and select the needed option.