WHMCS not sending emails

[jcdfusion]

Hi,

 

WHMCS suddenly stopped sending emails. I set it up using PHP mail. My own site uses it and emails are being sent, but WHMCS does not send ticket replies, cron job notifications, New orders, confirmations, invoices or anything except when I tried sending through pre-sale form, and it got sent to the junk folder anyway. Apart from the pre-sale question, no emails are being received neither in the inbox or spam folders.

 

What could be causing this, and how do I fix it?

 

Thanks