diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index ba61695229..c22b127f32 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -4325,6 +4325,8 @@ public class MessageHelper {
             // flag-keyword    = atom
             // atom            = 1*ATOM-CHAR
             // ATOM-CHAR       = <any CHAR except atom-specials>
+            // CHAR8           = %x01-ff ; any OCTET except NUL, %x00
+            // So, basically ISO 8859-1
             char kar = keyword.charAt(i);
             // atom-specials   = "(" / ")" / "{" / SP / CTL / list-wildcards / quoted-specials / resp-specials
             if (kar == '(' || kar == ')' || kar == '{' || kar == ' ' || Character.isISOControl(kar))