Hello Mandeep,

I'm looking into this some more. You mentioned that the emails are still delivered to the alias. Can you please tell me whether the sender still receives a bounce message? 

I don't believe so. The first image below is the rule I have setup, the second is the delivery report and the third one is from the smtp report. As you can see in the delivery report, only the ones that users have taught the system to mark as spam are being deleted (internal blocked senders filter) but the others are making it through. As I was taking these screenshots, it appears that the from address used is the same as the sent to address. So it is a self sending spam at the moment. Under SMTP In protocol settings, I checked and my settings for allow relay says nobody and the checkbox for "allow relay from authenticated users" is checked.

Mystery deepens.

Thanks