Mail sending blocked without a reason.

Mail sending blocked without a reason.

We have an account being blocked recently. When I try to send a mail the following message appears:

"Unable to connect SMTP server:smtpout.mail.zoho.com Reason:Authentication failed:554 5.1.8 Sender Address Blocked"

In my control panel / Troublesoot / blocked accounts there's no description telling why this account is blocked. When I try to unblock the account I get the message "Operation not permitted"

The account is a jira notifications account that  has been running for years with the same usage and now it's being blocked without reason.

We need to know why this account is blocked and also this account unblocked ASAP.

I've already sent several mails to support@zohomail.com without response.

Thanks,