diff --git a/app/src/main/java/eu/faircode/email/Helper.java b/app/src/main/java/eu/faircode/email/Helper.java
index 683096f2f9..28fa99be9c 100644
--- a/app/src/main/java/eu/faircode/email/Helper.java
+++ b/app/src/main/java/eu/faircode/email/Helper.java
@@ -741,6 +741,38 @@ public class Helper {
         return false;
     }
 
+    static boolean isUTF8(String text) {
+        // Get extended ASCII characters
+        byte[] octets = text.getBytes(StandardCharsets.ISO_8859_1);
+
+        for (int i = 0; i < octets.length; i++) {
+            int bytes;
+            if ((octets[i] & 0b10000000) == 0b00000000)
+                bytes = 1;
+            else if ((octets[i] & 0b11100000) == 0b11000000)
+                bytes = 2;
+            else if ((octets[i] & 0b11110000) == 0b11100000)
+                bytes = 3;
+            else if ((octets[i] & 0b11111000) == 0b11110000)
+                bytes = 4;
+            else if ((octets[i] & 0b11111100) == 0b11111000)
+                bytes = 5;
+            else if ((octets[i] & 0b11111110) == 0b11111100)
+                bytes = 6;
+            else
+                return false;
+
+            if (i + bytes > octets.length)
+                return false;
+
+            while (--bytes > 0)
+                if ((octets[++i] & 0b11000000) != 0b10000000)
+                    return false;
+        }
+
+        return true;
+    }
+
     // Files
 
     static String sanitizeFilename(String name) {
diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 52f2a4e834..00a87b9b0f 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -854,7 +854,10 @@ public class MessageHelper {
         if (header == null)
             return null;
 
-        header = new String(header.getBytes(StandardCharsets.ISO_8859_1));
+        if (!Helper.isUTF8(header)) {
+            Log.w("Converting header '" + name + "' to ISO8859-1");
+            header = new String(header.getBytes(StandardCharsets.ISO_8859_1), StandardCharsets.ISO_8859_1);
+        }
         Address[] addresses = InternetAddress.parseHeader(header, false);
 
         for (Address address : addresses) {
@@ -995,7 +998,10 @@ public class MessageHelper {
         if (subject == null)
             return null;
 
-        subject = new String(subject.getBytes(StandardCharsets.ISO_8859_1));
+        if (!Helper.isUTF8(subject)) {
+            Log.w("Converting subject to ISO8859-1");
+            subject = new String(subject.getBytes(StandardCharsets.ISO_8859_1), StandardCharsets.ISO_8859_1);
+        }
         subject = subject.replaceAll("\\?=\\r?\\n\\s+=\\?", "\\?==\\?");
         subject = MimeUtility.unfold(subject);
         subject = decodeMime(subject);