If you haven’t changed anything, then I’d imagine you’re using the default set up, which uses Amazon to send emails.
In this case, if Amazon has had previous bounce backs or rejections of emails, they are usually logged to prevent further issues. Are any of your clients blocked at the moment (see this guide for info)?
The other thing to check is the sent mail log. If you go to Account Settings >> All Settings >> Sent Mail Log, this will show you any emails sent from your account. It will also show whether they were sent or blocked. It will also show you if they’ve been opened too.