applicant doesn't receive confirmation mail

Some of our customers do not receive a confirmation email, when tey have submitted a form to us. It just occurs for a few of the customers, most of them do receive the email.

We have tried to look in the spam folder, nothing's there though.

We doublechecked that the emailaddress field is set to obligatory so that can't be the reason.

Or can the reason be that the customer's ISP mailserver rejects mail coming from your server?

Or????

Please help us here?

We do apologize for the inconvenience.

May we know the form in question?

Can you please share the submission ID in which the autoresponder email is not sending?

To get the submission ID, kindly follow this guide:https://www.jotform.com/help/549-where-to-find-the-submission-id

Looking forward for your response.