sts-before-send-email

/**
* This filter enables you to hook into the mail sending process and
* create your own method. You get all the information necessary.
* If you return something else than null the mail will not be send
* by the usual process.
*
* @since 1.0.0
*
* @param (null)     $continue         With the value null the mail will be send afterwards
* @param (string)     $to             Receiver Email
* @param (string)     $subject        Mail subject
* @param (string)     $body             HTML body
* @param (array)     $headers         Email Headers
* @param (array)     $attachments     Email Attachments
* @return (mixed)     $continue         With the value null the mail will be send afterwards
*/

This filter basically enables you to disable the usual functionality of sts_mail() to send a email. All necessary information, you need to send the email by your own will be send through this filter. If you return something else than null you tell sts_mail() not to send the email. If you do so, please return a boolean indicating if the email has been send or not.