If you’re using Google Workspace, Google will give you the appropriate DMARC, DKIM and SPF records to add to your DNS. The NS themselves should resolve the records and provide the recipient server with the values you’ve entered, thereby ensuring delivery.
Sure. But why would that matter when you're dealing with hostile 3rd party email providers that intentionally want to blackhole all email domains at Namecheap? But yes, just to clarify I do configure DMARC/DKIM/SPF and that works great for most cases.
I'm just describing what worked for me though in truth I don't know exactly how these hostile email providers actually determine the domain is hosted at Namecheap. My hunch is that they are using a lookup & finding the nameserver for the domain & have already blacklisted Namecheap's default free nameserver IP addresses. For whatever reason those same hostile email providers don't seem to be blacklisting Namecheap's paid nameserver but I think that sort of makes sense...
The larger issue is that Namecheap is known for cheap domains that scammers/spammers tend to buy in bulk & then use to spam with. Those same scammers/spammers aren't trying to spend extra money so they only ever use the default free Namecheap nameservers.