Paid Zoho Mail users cannot send email "Could not connect to SMTP host: smtpout.secureserver.net, port: 25, response: 554"

Paid Zoho Mail users cannot send email "Could not connect to SMTP host: smtpout.secureserver.net, port: 25, response: 554"

We have not been able to send email since ~4pm EST Dec 3 - yesterday (although we can receive). It is not related to our email host (we use GoDaddy) as I can set up the account information in Outlook and it sends and receives fine through there.

The error message I receive is:
Unable to send message; Reason: Could not connect to SMTP host: smtpout.secureserver.net, port: 25, response: 554

Per GoDaddy support, I have tried changing the port to 80 - which Zoho does not allow. I have also tried changing the port to 465 - which gives me the same error message as port 25.

We are paid Zoho Mail (and CRM) customers and are receiving no support. We rely on Zoho Mail for our business and are sitting dead in the water waiting for a response.  I sent in a support ticket over 12 hours ago and have had no response. I called last night and this morning and the message says "Mail support is not available." I left a message this morning and am waiting for a call back.