I'm not going to interfere with your contract with a different party that handles your mail or DNS if that's how you manage your mail or DNS.
I'm a nice guy so I'll signal a temporary error and if you arrange payment (or at least claim that you will arrange payment shortly), hopefully your correspondents will retry and you'll just have delays and not lost messages.
The remedy for failure to pay is to seek payment or sue, not to sabotage the client's business.
There's no reason to respond to questions or points nobody is making.