The EMailQueue is used by the mail service to queue requests to send mail. When a request is made to the mail service to send email, that request is queued on the EMailQueue. When the request is processed from the queue, the email is sent. The mail service also uses the queue to retry sending emails that failed to be sent. When a request to send an email fails, the mail service requires the request to be processed again after delaying for a period of time. By default, a request to send email is valid for 24 hours; if the mail service hasn't successfully processed the request by then, the mail service gives up trying to send the email for that request.