I sent an email from my new Zoho mail account to my gmail address.

It went through, although it landed in the spam folder.

But then when I tried replying to the email using my gmail account, the Email Delivery Subsystem blocked the mail.

This was the response I got;

Message blocked.

Your message to (Zoho mail account address) has been blocked. See technical details below for more information.

The response from the remote server was:

554 5.1.1: Recipient address rejected: undeliverable address: Mailbox might be disabled, full, or may not exist on the server. Reason: JFE030050

Please treat as urgent.