Outlook suddenly stopped syncing with my Galaxy S4

I got my new Galaxy S4 a couple of weeks ago and my Outlook email account was syncing with it without a problem until recently.  I just noticed I haven't received any new email on it since Sunday (three days ago), even after I manually selected the sync button.  I have checked my Outlook mailbox on my iMac and I have indeed received new mail.  Does anyone know the reason for this, or can someone please let me know how to fix this?