When you create an invoice, and you see a message that you're blocked from sending invoices to your client, please contact your client and ask them to update their vendor settings or whitelist your email address.
Via "Company Settings" > "Vendor Settings," your client can choose from whom they want to receive bills:
"Everyone except blocked vendors" means that any vendor can send a bill to the client. However, the client can block certain vendors. Once a client blocks a vendor, bills from this vendor won't be delivered to the client.
"No one except approved vendors" means that only bills from approved vendors will be delivered to the client. The client can approve vendors by adding their email address after clicking Approve a Vendor.
