Test users not receiving invitation email

[Dane_Ryan]

I have created a very simple test application using app sheet.
I am currently trying to share the app with some testers.

See the documentation below.

help.appsheet.com

Granting Application Access

How to give people access to your app

“If you would like the user to receive an invitation email with installation instructions, click the Send install link button at the bottom of the form. You can optionally customize the message by editing the Message to send text. If you would like to add them to the whitelist without sending them an email, click the Don’t Send Email button.”

I have followed these steps, but my users are not receiving any emails.

What could possibly be the reason for this?