Mail delivery over your local server uses PHP's native mail settings for email delivery. This may increase your risk of having emails marked as spam.

Mail delivery over a standard SMTP server uses PHPMailer over SMTP for email delivery.

Mandrill is a transactional email API by the makers of MailChimp.
Your API Key can be found on the Settings page of the Mandrill control panel.

It is advised that you verify your "sending domain" (the domain that you plan to use in the "From" address of your emails) via DKIM and SPF to reduce the risk of your email being marked as spam.

Mailgun is a transactional email API by Rackspace.
You must enter both your API Key (found on the landing page after logging in) and the domain you added to Mailgun that you plan to send emails from (you may use your Mailgun sandbox subdomain to send test emails).

It is required that you verify your "sending domain" (the domain that you plan to use in the "From" address of your emails) via DKIM and SPF to send more than 300 emails per day. It is also recommended even if you fall below that threshold as it will reduce the risk of your email being marked as spam.

Postmark is a transactional email API by the makers of Beanstalk.
You can find your API Key on the Credentials page of the Postmark server you wish to use.

It is advised that you verify your "sending domain" (the domain that you plan to use in the "From" address of your emails) via DKIM and SPF to reduce the risk of your email being marked as spam.

SendGrid is a transactional email delivery and management service.
You must enter both your API user and API key (the password that you use to log into sendgrid.com).