Usually, we can go to the Dashboard and view the errors under Tasks. This lists the emails with delivery errors sent through your email provider's server. In the delivery errors section, the delivery date, type, Ref No., Customer, error,and delivery method are found. You can change the email and resend it from there.
I've attached photos for visual references.
We can also go to your Audit Log if your Dashboard doesn't show any bounced emails. Here's how.
Click on the Gear icon.
Under Tools, select Audit Log.
Find the emailed transaction, then click the View button under the History column.
From there, you can check the Delivery Error.
If it's successfully sent, there'll be no error displayed. But if the email isn't successfully sent, then there'll be a "Bounced Email" status displayed beside it.
I’m also adding articles with detailed information on why email bounces and invoices dropped. It includes links on how to fix undelivered invoices.