If your customer is using webmail like Gmail and Yahoo Mail, they can try a different browser. This is to test if the issue only exists to the one they use. They can also do the basic browser troubleshooting steps, like clearing the cached images and files. Do not include cookies and passwords.
If they are using installed mail applications like Outlook, please contact our Support team from 6 AM to 6 PM PST during weekdays. That way, they can investigate this further. You can also try sending estimates to your email to see if you view it.
To contact our Support team, you'll want to click on Help and repeatedly search for the words need human. Then, you'll be given a guide to asking a question. Just click on See examples if it's displayed. Then, search for need human again, until you'll see an option that says I still need a human. When you click that, you'll have the Contact us option to reach out to Support.
If you have other questions, please don't hesitate to go back to this thread.