use utf-8 if unknown charset in get_header_unicode()

app/email_utils.py

@@ -603,6 +603,11 @@ def get_header_unicode(header: str):
             return decoded_string.decode(charset)
         except UnicodeDecodeError:
             LOG.warning("Cannot decode header %s", header)
+        except LookupError:  # charset is unknown, e.g.
+            LOG.exception(
+                "Cannot decode %s with %s, use utf-8", decoded_string, charset
+            )
+            return decoded_string.decode("utf-8")
 
     return header