Email to one address is bouncing (hosted ninja)

Been a user for several years and most of the time, it is great. Very few issues at all. However, I have recently had 3 emails to the same address Bounce. They show as bounces, and I got a notification on one. I can send email directly, using my email program to that same address. But how can I tell what is going on with these invoices getting bounced back. It is only one email address, and I only invoice them annually, so not a great BIG deal, although they had to follow-up with me to see why they had not received it.

And I do use the hosted (not self-) version. So I don’t know if there are any logs or anything.

You can click the ‘Email history’ button on the client overview page to check the email logs.