diff --git a/modules/admin/actions/client.ts b/modules/admin/actions/client.ts
index 754a907..b8a8884 100644
--- a/modules/admin/actions/client.ts
+++ b/modules/admin/actions/client.ts
@@ -55,13 +55,19 @@ export async function createClient(
         return { error: error.code === "email_exists" ? "Diese Email ist bereits vergeben." : "Client konnte nicht angelegt werden." }
     }
 
-    const mail = welcomeMail({ name, email, appUrl: await getAppUrl() });
-    const sent = await sendMail({
-        to: email,
-        ...mail,
-        type: "welcome",
-        idempotencyKey: `welcome-${data.user.id}`,
-    });
+    // Der Account existiert bereits: ein Mailfehler darf nicht verhindern, dass das Passwort angezeigt wird.
+    let welcomeMailSent = false;
+    try {
+        const sent = await sendMail({
+            to: email,
+            ...welcomeMail({ name, email, appUrl: await getAppUrl() }),
+            type: "welcome",
+            idempotencyKey: `welcome-${data.user.id}`,
+        });
+        welcomeMailSent = sent.ok;
+    } catch (mailError) {
+        console.error("[mail] Willkommensmail fehlgeschlagen:", mailError);
+    }
 
-    return { success: true, email, password, welcomeMailSent: sent.ok }
+    return { success: true, email, password, welcomeMailSent }
 }
\ No newline at end of file
diff --git a/modules/shared/actions/credentials.ts b/modules/shared/actions/credentials.ts
index c34be66..b81f67f 100644
--- a/modules/shared/actions/credentials.ts
+++ b/modules/shared/actions/credentials.ts
@@ -63,17 +63,20 @@ export async function createCredential(
 
     // Nur wenn wir den Zugang hinterlegen – legt der Kunde ihn selbst an, braucht er keine Mail.
     if (session.isAdmin) {
-        const appUrl = await getAppUrl();
         const serviceLabel = getServiceById(validatedFields.data.service).label;
         after(async () => {
-            const recipient = await getMailRecipient(profileId);
-            if (!recipient) return;
-            await sendMail({
-                to: recipient.email,
-                ...credentialAddedMail({ name: recipient.name, serviceLabel, appUrl }),
-                type: "credential",
-                idempotencyKey: `credential-${credentialId}`,
-            });
+            try {
+                const recipient = await getMailRecipient(profileId);
+                if (!recipient) return;
+                await sendMail({
+                    to: recipient.email,
+                    ...credentialAddedMail({ name: recipient.name, serviceLabel, appUrl: await getAppUrl() }),
+                    type: "credential",
+                    idempotencyKey: `credential-${credentialId}`,
+                });
+            } catch (error) {
+                console.error("[mail] Hinweis-Mail zum neuen Zugang fehlgeschlagen:", error);
+            }
         });
     }
 
diff --git a/modules/shared/actions/invoices.ts b/modules/shared/actions/invoices.ts
index f5b2442..a38e387 100644
--- a/modules/shared/actions/invoices.ts
+++ b/modules/shared/actions/invoices.ts
@@ -41,34 +41,43 @@ async function deliverInvoiceMail(profileId: string, clientId: string, invoiceId
     if (!invoice) return { error: "Rechnung nicht gefunden." };
     if (invoice.status === "cancelled") return { error: "Stornierte Rechnungen werden nicht verschickt." };
 
-    const [pdfData] = await getInvoicesForPdf([invoiceId]);
-    const recipient = await getMailRecipient(profileId);
-    if (!pdfData || !recipient) return { error: "Rechnung oder Empfänger konnte nicht geladen werden." };
+    // Die Rechnung ist beim Aufruf aus createInvoice schon gespeichert: Fehler hier dürfen nicht nach oben durchschlagen.
+    try {
+        const [pdfData] = await getInvoicesForPdf([invoiceId]);
+        const recipient = await getMailRecipient(profileId);
+        if (!pdfData || !recipient) return { error: "Rechnung oder Empfänger konnte nicht geladen werden." };
 
-    const pdf = await renderInvoicePdf(pdfData);
-    const mail = invoiceMail({
-        name: recipient.name,
-        invoiceNumber: pdfData.invoiceNumber,
-        invoiceDate: pdfData.invoiceDate,
-        dueDate: pdfData.dueDate,
-        grossTotal: invoiceTotals(pdfData.items).gross,
-        appUrl: await getAppUrl(),
-    });
+        const [pdf, appUrl] = await Promise.all([renderInvoicePdf(pdfData), getAppUrl()]);
+        const mail = invoiceMail({
+            name: recipient.name,
+            invoiceNumber: pdfData.invoiceNumber,
+            invoiceDate: pdfData.invoiceDate,
+            dueDate: pdfData.dueDate,
+            grossTotal: invoiceTotals(pdfData.items).gross,
+            appUrl,
+        });
 
-    const sent = await sendMail({
-        to: recipient.email,
-        ...mail,
-        type: "invoice",
-        attachments: [{ filename: invoiceFileName(pdfData.invoiceNumber), content: new Uint8Array(pdf), contentType: "application/pdf" }],
-    });
-    if (!sent.ok) return { error: sent.error };
+        const sent = await sendMail({
+            to: recipient.email,
+            ...mail,
+            type: "invoice",
+            attachments: [{ filename: invoiceFileName(pdfData.invoiceNumber), content: new Uint8Array(pdf), contentType: "application/pdf" }],
+        });
+        if (!sent.ok) return { error: sent.error };
 
-    await supabase
-        .from("invoices")
-        .update({ email_sent_at: new Date().toISOString(), email_sent_to: recipient.email })
-        .eq("id", invoiceId);
+        const { error: updateError } = await supabase
+            .from("invoices")
+            .update({ email_sent_at: new Date().toISOString(), email_sent_to: recipient.email })
+            .eq("id", invoiceId);
+        if (updateError) {
+            console.error(`[mail] Rechnung ${invoiceId} wurde versendet, Versandstatus aber nicht gespeichert:`, updateError.message);
+        }
 
-    return null;
+        return null;
+    } catch (error) {
+        console.error(`[mail] Rechnung ${invoiceId} konnte nicht versendet werden:`, error);
+        return { error: "E-Mail konnte nicht gesendet werden." };
+    }
 }
 
 export async function createInvoice(draft: InvoiceDraft, options: { sendEmail?: boolean } = {}): Promise<CreateInvoiceState> {
diff --git a/modules/shared/mail/app-url.ts b/modules/shared/mail/app-url.ts
index 0a21b3e..9aa2fd2 100644
--- a/modules/shared/mail/app-url.ts
+++ b/modules/shared/mail/app-url.ts
@@ -1,15 +1,19 @@
 import "server-only"
 import { headers } from "next/headers"
 
-// Basis-URL für Links in Mails. APP_URL hat Vorrang, auf Vercel greift die Produktionsdomain, sonst der aktuelle Request.
+// Basis-URL für Links in Mails. Nie aus Request-Headern in Produktion: Host/X-Forwarded-Host sind vom Client fälschbar.
+// Reihenfolge: APP_URL, Vercel-Produktionsdomain, Vercel-Deployment-URL (Preview), lokal der Request-Host.
 export async function getAppUrl() {
     if (process.env.APP_URL) return process.env.APP_URL.replace(/\/$/, "")
     if (process.env.VERCEL_ENV === "production" && process.env.VERCEL_PROJECT_PRODUCTION_URL) {
         return `https://${process.env.VERCEL_PROJECT_PRODUCTION_URL}`
     }
+    if (process.env.VERCEL_URL) return `https://${process.env.VERCEL_URL}`
 
-    const headerList = await headers()
-    const host = headerList.get("x-forwarded-host") ?? headerList.get("host") ?? "localhost:3000"
-    const protocol = headerList.get("x-forwarded-proto") ?? (host.startsWith("localhost") ? "http" : "https")
-    return `${protocol}://${host}`
+    if (process.env.NODE_ENV === "development") {
+        const host = (await headers()).get("host") ?? "localhost:3000"
+        return `http://${host}`
+    }
+
+    throw new Error("APP_URL ist nicht gesetzt – ohne feste Basis-URL werden keine Mail-Links erzeugt.")
 }