Manual:Hooks/UserMailerTransformMessage

Details

 * $to: array of MailAddress objects of the targets
 * $from: MailAddress of the sender
 * &$subject: email subject (not MIME encoded)
 * &$headers: email headers (except To: and Subject:) as an array of header name => value pairs
 * &$body: email body (in MIME format) as a string
 * &$error: should be set to an error message string, by default false