Mailer – choose whether you want to use the Default PHP mail() function, a Gmail/G Suite account, Mailgun, SendGrid or other SMTP servers. Choosing the Default mode would send the messages using the PHP mail() function without SMTP authentication. The Gmail/G Suite, Mailgun and SendGrid options require additional info such as Client ID and Client Secret or API key that should be obtained by the respective provider. The other SMTP server option would prompt you to enter additional configuration details which we would describe below.