email notifications are not send

I’m having a problem with emails which are supposed to be automatically being send to my clients in my shopify store. For example: my husband purchased an item in my store with his own private email address, details and cards as a try out to see if everything was working correctly. The purchase went through OK. I got notified that someone had purchased something in my store. I then fulfilled the purchase in my store via Oberlo with all my details and store payment method. That went fine also. Then a confirmation email was send to my store saying that the email account of my husband didn't exist so they couldn't deliver the confirmation email. When I fulfill an order there should be an email send automatically to the client who purchased in my store. It should first of all not go to my store inbox. Secondly the email account of my husband does exist he has been using it for over 20 years. So I guess my clients are not getting their confirmation emails or tracking codes for that matter. It said exactly this: THIS IS THE MAIL SYSTEM AT HOST [login to view URL] i'M SORRY TO HAVE INFORM YOU THAT YOUR MESSAGE COULD NOT BE DELIVERED TO ONE OR MORE RECIPIENTS. IT'S ATTACHED BELOW FOR FUTHER ASSITENCE PLEASE SENDMAILTO POSTMASTER. IF YOU DO SO, PLEASE INCLUDE THISPROBLEM REPORT. OU CAN DELETE YOUR OWN TEXT FROM THE ATTACHED RETURNED MESSAGE. The forwarded message was just the order [login to view URL] it seems that my automatically confirmation notices which are send to my clients after I fulfill their orders end up in my store's email box instead of the client. That is if they even can be delivered.