Hi PBTannie,
Thank you for your post, and welcome to the community forum!
I apologize for the inconvenience if you are unable to send emails and invoices to your customers as they are being rejected. Depending on the security settings at your customers' end, the emails received may be put in the spam or junk folder. It will also help to have your customers add noreply@apps.myob.com to their whitelist to make sure the emails are not blocked. If the issue persists, you can send me the email address via private message of the customers who are not able to receive the email, so we can check and assist you further.
Feel free to write a post if you still need further assistance.
Cheers,
Jem