Go to Analytics and click the Bounces tab. This shows how many emails sent to your aliases couldn't be delivered.
The three stats
Total bounces — how many emails bounced in the selected period.
Bounce rate — bounces as a percentage of total emails received. A high bounce rate means a lot of emails are being blocked or failing.
Bounce types — a breakdown of why emails bounced.
Common bounce reasons
Deactivated alias — the most common. Someone sent to an alias that was turned off. The sender receives a delivery failure notification.
Destination unreachable — the destination the alias forwards to couldn't accept the email. This could be a full mailbox, a temporary outage, or the destination having been removed.
Permission blocked — the alias's permission mode rejected the sending address.
Invalid alias — the email was sent to an address that doesn't exist on your account (typo from the sender, or an alias you deleted).
What a healthy bounce rate looks like
A low bounce rate (under 5%) is normal — some bounces are expected from deactivated aliases or spam attempts to random addresses. A high bounce rate worth investigating would be 20%+ sustained over time, which could indicate a misconfiguration or a destination issue.
Taking action on bounces
If bounce rate is high, check the Alias Performance tab to find which aliases have the most blocked email, then investigate those specifically.