Pastel invoice not sending from Outlook when closed

Hi all. I have previously been able to email invoices from Pastel through Outlook 2013 without Outlook being open. Now suddenly it goes to the outbox in Outlook and only sends as soon as I open Outlook, and even when I open Outlook it sits in the outbox for about 2 minutes before moving to the send items. Does anyone know what could be the cause of this hold on the invoice? (I do not get any errors in Pastel sending the email)