Reports has a List Health section. Pick any date range and you see unsubscribes, bounces and spam complaints, each with a trend against the previous period. A rise in unsubscribes shows red, a fall shows green.
Underneath, the latest events with the reason and which provider reported them.
The benchmarks to keep in mind: complaints under 0.1 percent of what you send, bounces under 2 percent. Go past those for long and inbox providers start sending you to spam.
Everyone counted here is automatically protected from all future sends. To browse, search, add or remove those addresses, or to import the unsubscribe file from your old autoresponder, use the suppression manager on the Deliverability tab. Import that file BEFORE you import your subscribers, so people who already left never get mailed again.
Your sending provider can report bounces and complaints back to you automatically. On the Sending tab each provider shows its own webhook URL with a copy button. Paste it into your provider's webhook settings and enable the bounce, spam and unsubscribe events. The secret in that URL is what stops anyone else feeding you fake events, so treat it like a password.