diff --git a/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/CsvImportButton.tsx b/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/CsvImportButton.tsx
index bc2d7e3..3cd3dad 100644
--- a/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/CsvImportButton.tsx
+++ b/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/CsvImportButton.tsx
@@ -111,6 +111,11 @@ export function CsvImportButton({ onImported }: CsvImportButtonProps) {
       {/* Import mode inline dialog (below button, per UI-SPEC) */}
       {showDialog && selectedFile && (
         <div className="absolute right-0 top-10 z-30 w-80 rounded border border-border bg-card p-4 shadow-md">
+          <div className="mb-3 rounded bg-muted/50 px-3 py-2 text-xs text-muted-foreground space-y-0.5">
+            <p>{t('csvFormatHelp')}</p>
+            <p className="font-mono">{t('csvFormatExample')}</p>
+          </div>
+
           <p className="text-sm font-medium text-foreground mb-3">
             {t('importMode')}
           </p>
diff --git a/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/InboxConfigForm.tsx b/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/InboxConfigForm.tsx
index 8784ac6..c865bc3 100644
--- a/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/InboxConfigForm.tsx
+++ b/apps/web/src/app/(portal)/modules/dkv-fleet/settings/components/InboxConfigForm.tsx
@@ -507,20 +507,36 @@ export function InboxConfigForm() {
 
       {/* Inline test feedback */}
       {testResult !== null && (
-        <p
-          className="text-sm"
-          style={{
-            color: testResult.success
-              ? 'oklch(0.40 0.15 148)'
-              : 'var(--destructive)',
-          }}
-        >
-          {testResult.success
-            ? t('form.testSuccess')
-            : t('form.testFailed', {
-                error: testResult.message ?? 'Unbekannter Fehler',
-              })}
-        </p>
+        <div className="space-y-1">
+          <p
+            className="text-sm"
+            style={{
+              color: testResult.success
+                ? 'oklch(0.40 0.15 148)'
+                : 'var(--destructive)',
+            }}
+          >
+            {testResult.success
+              ? t('form.testSuccess')
+              : t('form.testFailed', {
+                  error: testResult.message ?? 'Unbekannter Fehler',
+                })}
+          </p>
+          {/* 401 hint for Exchange */}
+          {!testResult.success &&
+            form.protocol === 'exchange' &&
+            testResult.message?.includes('401') && (
+              <div className="rounded bg-muted/50 px-3 py-2 text-xs text-muted-foreground space-y-1">
+                <p className="font-medium">{t('form.exchangeAuthHint')}</p>
+                <ul className="list-disc list-inside space-y-0.5">
+                  <li>{t('form.exchangeAuthHint1')}</li>
+                  <li>{t('form.exchangeAuthHint2')}</li>
+                  <li>{t('form.exchangeAuthHint3')}</li>
+                  <li>{t('form.exchangeAuthHint4')}</li>
+                </ul>
+              </div>
+            )}
+        </div>
       )}
 
       {/* Inline save feedback */}
diff --git a/apps/web/src/messages/de.json b/apps/web/src/messages/de.json
index eeeee96..dd19bb8 100644
--- a/apps/web/src/messages/de.json
+++ b/apps/web/src/messages/de.json
@@ -296,6 +296,8 @@
     "deleteVehicle": "Fahrzeug loeschen",
     "deleteVehicleBody": "Das Fahrzeug {plate} wird dauerhaft aus der Stammdatenliste entfernt. Diese Aktion kann nicht rueckgaengig gemacht werden.",
     "importCsv": "CSV importieren",
+    "csvFormatHelp": "CSV-Format (Semikolon): Kennzeichen;Marke;Modell;Fahrer",
+    "csvFormatExample": "Beispiel: GP-JL 728E;Mercedes;GLC 300;Max Mustermann",
     "importMode": "Importmodus",
     "importMerge": "Zusammenfuehren — neue Fahrzeuge hinzufuegen, vorhandene behalten",
     "importReplace": "Ersetzen — alle vorhandenen Fahrzeuge loeschen und neu importieren",
@@ -337,7 +339,12 @@
       "cancel": "Bearbeitung abbrechen",
       "cancelDialog": "Abbrechen",
       "showPassword": "Passwort anzeigen",
-      "hidePassword": "Passwort verbergen"
+      "hidePassword": "Passwort verbergen",
+      "exchangeAuthHint": "401 — moegliche Ursachen:",
+      "exchangeAuthHint1": "Falsches Passwort oder Benutzername",
+      "exchangeAuthHint2": "Domain: Windows-Kurzname (z.B. FIRMA, nicht firma.de)",
+      "exchangeAuthHint3": "Benutzername ohne Domain-Praefix (z.B. 'max.muster', nicht 'FIRMA\\max.muster')",
+      "exchangeAuthHint4": "Office 365 / Exchange Online: EWS-Basisauthentifizierung seit Okt. 2022 deaktiviert — nicht unterstuetzt"
     },
     "errors": {
       "pollFailed": "Posteingang konnte nicht geprueft werden. Ueberpruefe die Konfiguration unter 'Moduleinstellungen > Posteingang'.",
diff --git a/apps/web/src/messages/en.json b/apps/web/src/messages/en.json
index 9523932..6eebd96 100644
--- a/apps/web/src/messages/en.json
+++ b/apps/web/src/messages/en.json
@@ -296,6 +296,8 @@
     "deleteVehicle": "Delete Vehicle",
     "deleteVehicleBody": "Vehicle {plate} will be permanently removed from the master list. This action cannot be undone.",
     "importCsv": "Import CSV",
+    "csvFormatHelp": "CSV format (semicolon): Kennzeichen;Marke;Modell;Fahrer",
+    "csvFormatExample": "Example: GP-JL 728E;Mercedes;GLC 300;Max Mustermann",
     "importMode": "Import Mode",
     "importMerge": "Merge — add new vehicles, keep existing",
     "importReplace": "Replace — delete all existing vehicles and re-import",
@@ -337,7 +339,12 @@
       "cancel": "Cancel Editing",
       "cancelDialog": "Cancel",
       "showPassword": "Show password",
-      "hidePassword": "Hide password"
+      "hidePassword": "Hide password",
+      "exchangeAuthHint": "401 — possible causes:",
+      "exchangeAuthHint1": "Wrong password or username",
+      "exchangeAuthHint2": "Domain: Windows short name (e.g. COMPANY, not company.com)",
+      "exchangeAuthHint3": "Username without domain prefix (e.g. 'max.muster', not 'COMPANY\\max.muster')",
+      "exchangeAuthHint4": "Office 365 / Exchange Online: EWS basic auth disabled since Oct 2022 — not supported"
     },
     "errors": {
       "pollFailed": "Could not check inbox. Review the configuration under 'Module Settings > Inbox'.",