User not receiving subscribed topics

I have one user on my system that’s not receiving the emails of new posts. I confirmed they are not being sent (via email sent) but I can’t figure out why he’s the only one.

Any ideas on what to check?

The first place to look is on your site’s Admin / Emails / Bounced page. Try entering the user’s email address into that page’s “To Address” search filter. If any bounces show up, you should be able to see the bounce reason by clicking the link that will be shown in the “Email Type” field.