Email Hell!

Need some cross platform email problem resolution. We use Quickbooks to send customer (invoices, estimates, etc...), vendor (POs, etc...) and employee (pay stubs). Email is sent via a noreply address on a VPS under second domain on the server. Primary domain on server is managed by Google (workspace) while secondary domain is not. Problems - Paystub document type - Quickbooks sending will fail with Paystub with Firewall/password error is being sent by non-primary/non-google domain to primary/google managed domain. If sending to outside domain it will say it has processed but often/all times not received. Other document types - often received but intermittently at least not received. No rhyme or reason. Understand that this is probably a setting conflict issue somewhere between the google and non-google domains on the VPS but strange that QBs is erroring out with just the non-google to google managed domains. Prefer someone we can speak through with this not just chat.