How to determine block/reject reasons?

I know of about 20 email addresses that reject emails from me. I'm trying to determine the reason so I tried sending a test email to them. Zoho blocked my account after my sending to 8 of them. Yes, I know I'm going to get errors one each of them; that's intentional. How do I perform this kind of task without Zoho determining that I'm some sort of security threat?