Excessive Spam, but only with Zoho....why?

Excessive Spam, but only with Zoho....why?

I've been experimenting with using a different mail service other than Zoho.  Not because I don't love Zoho, I do.  There are integration reasons with other software I use that have made me explore my options.  

2 times over the past month I have changed my MX records to something different than Zoho during the experiment.  While it doesn't look like I will be moving away from Zoho, I did notice during those 2 times that the amount of spam I received plummeted.  I typically get about 100 or more spam messages a day.  With the 2 other providers I tried I was only getting 4 or 5.  More astonishing is that even though my MX records were changed, my Zoho account was still getting spam email in the spam folder.  All normal mail was routed to where I had my MX records changed to. 

Why is that?

Is that because the spam is coming from Zoho accounts or dare I say from Zoho itself?