NopCommerce v3.5 only sends email to same hosting provider (GoDaddy)

I have a client that has had no problems allowing people to register, log in, place orders, recover passwords, and all associated email communications occurred without errors.

About 2 weeks ago, it seems that the system will only send emails to an email on the same hosting provider (GoDaddy). Other email addresses never receive the emails, and no errors are generated on either end.

System Log File shows no email errors, and System Message Queue shows no failed messages.

Using the standard GoDaddy configuration:
relay-hosting.secureserver.net
port 25, no SSL, no default credentials

Sending test emails reveal no errors; however, only other GoDaddy-based email addresses receive messages. Non-GoDaddy addresses (tx.rr.com, hotmail, and gmail) never receive any messages or errors.

Anyone have any ideas why it would suddenly fail in such a specific manner?

This seems to be more of a GoDaddy problem. You can use SendGrid if that's the case. https://sendgrid.com/

do raise a ticket on godaddy site, let them work with you to find the issues.

btw end of this month we will launch our new platform for edm, if you are interested, drop me a pm

GoDaddy claimed that the word "validation" might have been causing the issue. Although this is true on the actual customer confirmation email, it's not true on the "send test message" functionality from the administration backend, and both fail in the same way.

Tried changing the sending email to a Spectrum address and then a Hotmail address, and I got the same results each time... GoDaddy email received it fine, non-GoDaddy email never received it.