return only bounce report that can be decoded

app/email_utils.py

@@ -644,6 +644,13 @@ def get_mailbox_bounce_info(bounce_report: Message) -> Optional[Message]:
             if not part["content-transfer-encoding"]:
                 LOG.w("add missing content-transfer-encoding header")
                 part["content-transfer-encoding"] = "7bit"
+
+            try:
+                part.as_bytes().decode()
+            except UnicodeDecodeError:
+                LOG.w("cannot use this bounce report")
+                return
+            else:
                 return part