If your customers rarely receive invoices email, have them check their junk or spam folder. You'll want to try a few steps by sending the invoice to your own email to check if you'll be able to receive it. If so, there's nothing wrong with how you send the invoice.
Furthermore, let your customer use a private window when accessing the email. This will help us isolate the issue because this mode will not save site data.
Here are the keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
They can also clear their browser's cache to speed up the loading process and for the system to start fresh.
For your reference, I have these help articles handy in case you have any question in QuickBooks Self-Employed.
Keep in touch if you have further questions. As always, we're right here to help you.
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