The issue is already addressed in the current thread.
Please check of your mail settings,
You might be using the default PHP mail to send emails which does not work properly in some scenarios.
We suggest you to please use SMTP method for sending emails.
To check and update Mail settings,
Log in to the QloApps back office > Advanced Parameters > E-mail tab.
In this tab you can use update SMTP server details to use SMTP instead of PHP mail for sending email.
After saving the details, you can try out to send a test email as well.