My customer complaint they do not receive the email order. So I have to manually send them the invoice from the admin account.
What cloud be the issue?
This question has been asked but I don't see any answers that reflect my experience. For me the issue has been narrowed down to a few situation. First off, check your mail logs to see if the email was actually sent. Sometimes you get good feedback and an exact reason why the email didn't send.
So really it comes down to two areas. The origin; it isn't sending from your server or hits a fatal error in Magento. Or the destination, a third party blacklist, or the final server is filtering out or hiding the email.
If its not sending at all, check your Magento and PHP error logs. If its not sending invoices only, but password reset, etc., work, then its almost certainly Magento.