Why my e-mail block?

Hello,
I have a problem, I loved the zoho mail, I use it in my host company as main email, I use it to convert all my alias emails into a single receiving box, and also to trigger my emails via smtp, my platform is whmcs that trigger emails for new orders, invoice reminders, new registration, password recovery, and so on. Nothing from email marketing, just emails for running the company, I hired zoho mail for this purpose, unfortunately I had my email blocked today, I did not understand why