diff --git a/modoboa/amavis/utils.py b/modoboa/amavis/utils.py
index d829eb02d..c3a210418 100644
--- a/modoboa/amavis/utils.py
+++ b/modoboa/amavis/utils.py
@@ -37,7 +37,7 @@ def smart_str(value, *args, **kwargs):
     return django_smart_str(value, *args, **kwargs)
 
 
-def fix_utf8_encoding(value):
+def fix_utf8_encoding(value: str) -> str:
     """Fix utf-8 strings that contain utf-8 escaped characters.
 
     msgs.from_addr and msgs.subject potentialy contain badly escaped utf-8
@@ -58,12 +58,13 @@ def fix_utf8_encoding(value):
         value = bytes_value.decode("utf-8")
     except UnicodeDecodeError:
         encoding = chardet.detect(bytes_value)
-        try:
-            value = bytes_value.decode(encoding["encoding"], "replace")
-        except (TypeError, UnicodeDecodeError):
-            # ??? use the original value, we've done our best to try and
-            # convert it to a clean utf-8 string.
-            pass
+        if encoding["confidence"] > 50.0:
+            try:
+                value = bytes_value.decode(encoding["encoding"], "replace")
+            except (TypeError, UnicodeDecodeError):
+                # ??? use the original value, we've done our best to try and
+                # convert it to a clean utf-8 string.
+                pass
 
     return value
 
diff --git a/modoboa/maillog/graphics.py b/modoboa/maillog/graphics.py
index 4596a692b..a4b537778 100644
--- a/modoboa/maillog/graphics.py
+++ b/modoboa/maillog/graphics.py
@@ -41,16 +41,16 @@ class Curve:
         return [
             f"DEF:{self.dsname}={rrdfile}:{self.dsname}:{self.cfunc}",
             f"CDEF:{self.dsname}pm={self.dsname},UN,0,{self.dsname},IF,60,*",
-            f'XPORT:{self.dsname}pm:"{self.legend}"',
+            f"XPORT:{self.dsname}pm:'{self.legend}'",
         ]
 
 
 class Graphic:
     """Graphic."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         """Constructor."""
-        self._curves = []
+        self._curves: list[Curve] = []
         try:
             order = self.order
         except AttributeError:
@@ -131,9 +131,9 @@ class Graphic:
 class GraphicSet:
     """A set of graphics."""
 
-    domain_selector = False
-    title = None
-    _graphics = []
+    domain_selector: bool = False
+    title: str | None = None
+    _graphics: list[Graphic] = []
 
     def __init__(self, instances=None):
         if instances is None: