Unable to send email (554 5.1.8 Sender Address Blocked)

Unable to send email (554 5.1.8 Sender Address Blocked)

Hi,

Since last monday (07/26/2017) I've been unable to send emails through my account.

I searched this forum for a solution and stumbled upon this link https://help.zoho.com/portal/en/community/topic/wordpress-smtp-not-working-dkim-spf-verified-remote-email-routing-enabled-yet.
After completing the 8 steps proposed by Mr. Barath KR I got the message "554 5.1.8 Sender Address Blocked".

I ask you to unblock it, and would like to know why that has happened?

Regards,
Vooozer