Let me share some troubleshooting steps to get to the bottom of these missing clients and test emails. Here are some possible reasons why you don't receive them:
The emails might have been marked as spam.
You are using a custom email domain or if you're using an email client (Outlook, Exchange, AOL, Thunderbird, etc.) which results in your email taking longer than web emails.
Your internet service or domain provider block their email address.
To start, check your Spam or Junk Mail folders to verify if the emails were moved there. If you don't see it, try clearing and re-entering your email address in the settings to refresh the system.
From the Gear icon, select Account and Settings.
Select the Company tab at the left pane and tick the Pencil icon in the Contact Info section.
Remove the email address, then re-enter it.
Hit Save and Done.
Once done, send yourself a test invoice to check if it works. If the email still doesn't appear, there may be some security settings that need to be set in your email server. I suggest you verify with your service provider to evaluate your options or use a different email address that has a different domain.
For additional reference, I've attached some articles you can use to make sure you receive important messages from Intuit, as well as how to let your customers pay their invoices online:
I am not receiving my clients email nor am I receiving test invoice email to me. Both using QBO. Please advise.
Our IT team ran a test and it seems like the issue is related to the SPF-records. I stopped receiving emails in early May, after receiving them for years.
I presume that they did a server or provider move and have not adjusted their SPF-records yet. This will cause problem with a lot of other customers, whose mail-servers are doing SPF record checking of incoming emails to prevent spam.