diff --git a/app/(main)/[locale]/page.tsx b/app/(main)/[locale]/page.tsx
index 0db35bc3..f0ae5a56 100644
--- a/app/(main)/[locale]/page.tsx
+++ b/app/(main)/[locale]/page.tsx
@@ -70,7 +70,7 @@ import { AppTopBannerSlot } from "@/components/plugins/app-top-banner-slot";
 import { useThemeStore } from "@/stores/theme-store";
 import { consumePendingMailto, subscribeToPendingMailto } from "@/lib/protocol-handlers/session";
 import type { ParsedMailto } from "@/lib/protocol-handlers/mailto";
-import { plainTextToComposerBody } from "@/lib/email-composer-utils";
+import { plainTextToComposerBody, getQuoteBodies } from "@/lib/email-composer-utils";
 import { appLifecycleHooks, uiHooks, routerHooks, toastHooks, emailHooks } from "@/lib/plugin-hooks";
 import { emailToReadView } from "@/lib/plugin-projection";
 import { buildQuoteHeader } from "@/lib/quote-header";
@@ -787,8 +787,7 @@ export default function Home() {
       cc: selectedEmail.cc,
       bcc: selectedEmail.bcc,
       subject: selectedEmail.subject,
-      body: selectedEmail.bodyValues?.[selectedEmail.textBody?.[0]?.partId || '']?.value || selectedEmail.preview || '',
-      htmlBody: selectedEmail.bodyValues?.[selectedEmail.htmlBody?.[0]?.partId || '']?.value || undefined,
+      ...getQuoteBodies(selectedEmail),
       receivedAt: selectedEmail.receivedAt,
       attachments: selectedEmail.attachments,
       messageId: selectedEmail.messageId,
@@ -1387,8 +1386,13 @@ export default function Home() {
     const bodyText = draft.bodyValues
       ? Object.values(draft.bodyValues).map(v => v.value).join('\n')
       : '';
-    const htmlBody = draft.htmlBody?.[0]?.partId && draft.bodyValues?.[draft.htmlBody[0].partId]
-      ? draft.bodyValues[draft.htmlBody[0].partId].value
+    // A plain-text-only draft lists its text/plain part under htmlBody
+    // (RFC 8621 § 4.1.4 fallback) - only treat it as HTML when it really is.
+    const draftHtmlPart = draft.htmlBody?.[0];
+    const htmlBody = draftHtmlPart?.partId
+      && (!draftHtmlPart.type || draftHtmlPart.type.toLowerCase() === 'text/html')
+      && draft.bodyValues?.[draftHtmlPart.partId]
+      ? draft.bodyValues[draftHtmlPart.partId].value
       : undefined;
 
     // Try to find the identity that matches the draft's from address to preserve it
@@ -3300,8 +3304,7 @@ export default function Home() {
                     cc: selectedEmail.cc,
                     bcc: selectedEmail.bcc,
                     subject: selectedEmail.subject,
-                    body: selectedEmail.bodyValues?.[selectedEmail.textBody?.[0]?.partId || '']?.value || selectedEmail.preview || '',
-                    htmlBody: selectedEmail.bodyValues?.[selectedEmail.htmlBody?.[0]?.partId || '']?.value || undefined,
+                    ...getQuoteBodies(selectedEmail),
                     receivedAt: selectedEmail.receivedAt,
                     attachments: selectedEmail.attachments,
                     messageId: selectedEmail.messageId,
diff --git a/components/pro/pro-email-tab-body.tsx b/components/pro/pro-email-tab-body.tsx
index 2094963e..bd7dfdfc 100644
--- a/components/pro/pro-email-tab-body.tsx
+++ b/components/pro/pro-email-tab-body.tsx
@@ -12,6 +12,7 @@ import { toast } from "@/stores/toast-store";
 import { useProTabStore, type ProEmailTabData, type ProReplyContext } from "@/stores/pro-tab-store";
 import type { Email } from "@/lib/jmap/types";
 import { buildReplySubject, buildForwardSubject } from "@/lib/subject-prefix";
+import { getQuoteBodies } from "@/lib/email-composer-utils";
 
 interface ProEmailTabBodyProps {
   tabId: string;
@@ -19,8 +20,6 @@ interface ProEmailTabBodyProps {
 }
 
 function buildReplyContext(email: Email): ProReplyContext {
-  const textPartId = email.textBody?.[0]?.partId ?? '';
-  const htmlPartId = email.htmlBody?.[0]?.partId ?? '';
   return {
     from: email.from,
     replyToAddresses: email.replyTo,
@@ -28,8 +27,7 @@ function buildReplyContext(email: Email): ProReplyContext {
     cc: email.cc,
     bcc: email.bcc,
     subject: email.subject,
-    body: email.bodyValues?.[textPartId]?.value || email.preview || '',
-    htmlBody: email.bodyValues?.[htmlPartId]?.value || undefined,
+    ...getQuoteBodies(email),
     receivedAt: email.receivedAt,
     accountId: email.accountId,
     attachments: email.attachments,
@@ -235,8 +233,13 @@ export function ProEmailTabBody({ tabId, data }: ProEmailTabBodyProps) {
     const bodyText = email.bodyValues
       ? Object.values(email.bodyValues).map((v) => v.value).join('\n')
       : '';
-    const htmlBody = email.htmlBody?.[0]?.partId && email.bodyValues?.[email.htmlBody[0].partId]
-      ? email.bodyValues[email.htmlBody[0].partId].value
+    // A plain-text-only draft lists its text/plain part under htmlBody
+    // (RFC 8621 § 4.1.4 fallback) - only treat it as HTML when it really is.
+    const draftHtmlPart = email.htmlBody?.[0];
+    const htmlBody = draftHtmlPart?.partId
+      && (!draftHtmlPart.type || draftHtmlPart.type.toLowerCase() === 'text/html')
+      && email.bodyValues?.[draftHtmlPart.partId]
+      ? email.bodyValues[draftHtmlPart.partId].value
       : undefined;
 
     // Preserve the identity that matches the draft's From address.
diff --git a/lib/__tests__/email-composer-utils.test.ts b/lib/__tests__/email-composer-utils.test.ts
index 5d40b7f0..b3a3b202 100644
--- a/lib/__tests__/email-composer-utils.test.ts
+++ b/lib/__tests__/email-composer-utils.test.ts
@@ -14,6 +14,7 @@ import {
   extractUserAuthoredText,
   formatRecipientEntry,
   expandRecipients,
+  getQuoteBodies,
 } from '../email-composer-utils';
 
 const FORWARDED_SEPARATOR = "---------- Forwarded message ----------";
@@ -477,3 +478,47 @@ describe('contact group recipients (RFC 5322 group syntax)', () => {
     expect(expandRecipients([{ name: 'X', email: 'x@y.z' }])).toEqual([{ name: 'X', email: 'x@y.z' }]);
   });
 });
+
+describe("getQuoteBodies", () => {
+  const part = (partId: string, type: string) => ({ partId, blobId: "b", size: 1, type });
+
+  it("converts an HTML-only message's shared part into readable text (#649)", () => {
+    const { body, htmlBody } = getQuoteBodies({
+      textBody: [part("1", "text/html")],
+      htmlBody: [part("1", "text/html")],
+      bodyValues: { "1": { value: "<p>Hallo Jonas.</p><p>Zeile zwei<br>und drei</p>" } },
+    });
+    expect(body).toBe("Hallo Jonas.\n\nZeile zwei\nund drei");
+    expect(htmlBody).toContain("<p>Hallo Jonas.</p>");
+  });
+
+  it("drops htmlBody when it is really the text/plain part (#649)", () => {
+    const text = "Hallo Herr Test,\n\ndas ist eine Test-Email.\n\nBeste Grüße";
+    const { body, htmlBody } = getQuoteBodies({
+      textBody: [part("1", "text/plain")],
+      htmlBody: [part("1", "text/plain")],
+      bodyValues: { "1": { value: text } },
+    });
+    expect(body).toBe(text);
+    expect(htmlBody).toBeUndefined();
+  });
+
+  it("passes both parts through when the message has real alternatives", () => {
+    const { body, htmlBody } = getQuoteBodies({
+      textBody: [part("t", "text/plain")],
+      htmlBody: [part("h", "text/html")],
+      bodyValues: {
+        t: { value: "plain version" },
+        h: { value: "<p>html version</p>" },
+      },
+    });
+    expect(body).toBe("plain version");
+    expect(htmlBody).toBe("<p>html version</p>");
+  });
+
+  it("falls back to the preview when body values are missing", () => {
+    const { body, htmlBody } = getQuoteBodies({ preview: "preview text" });
+    expect(body).toBe("preview text");
+    expect(htmlBody).toBeUndefined();
+  });
+});
diff --git a/lib/email-composer-utils.ts b/lib/email-composer-utils.ts
index 1ac83c19..57be4a0e 100644
--- a/lib/email-composer-utils.ts
+++ b/lib/email-composer-utils.ts
@@ -2,6 +2,7 @@ import { isValidEmail } from "@/lib/validation";
 import { htmlToPlainText } from "@/lib/html-to-text";
 import { emailHooks } from "@/lib/plugin-hooks";
 import { Ellipsis, Lock, TriangleAlert } from "lucide-react";
+import type { Email } from "@/lib/jmap/types";
 
 const HTML_ESCAPE_MAP = {
   "&": "&amp;",
@@ -17,6 +18,41 @@ function escapeHtml(value: string): string {
   );
 }
 
+/**
+ * Picks the plain-text and HTML bodies of an original message for seeding a
+ * reply/forward quote.
+ *
+ * Per RFC 8621 § 4.1.4 a message with only one body variant exposes that
+ * single part in BOTH `textBody` and `htmlBody`. So for an HTML-only message
+ * `textBody[0]` is the raw text/html source, and for a plain-text-only
+ * message `htmlBody[0]` is the text/plain part. Quoting either verbatim
+ * breaks the reply (#649): raw HTML tags end up in a plain-text quote, and
+ * plain text rendered as HTML collapses all newlines. Route by each part's
+ * actual MIME type instead: HTML listed under textBody is converted to
+ * readable text, and plain text listed under htmlBody is dropped so the
+ * composer's text path (escape + <br>) renders it.
+ */
+export function getQuoteBodies(
+  email: Pick<Email, "textBody" | "htmlBody" | "bodyValues" | "preview">
+): { body: string; htmlBody?: string } {
+  const textPart = email.textBody?.[0];
+  const htmlPart = email.htmlBody?.[0];
+  const textValue = textPart ? email.bodyValues?.[textPart.partId]?.value : undefined;
+  const htmlValue = htmlPart ? email.bodyValues?.[htmlPart.partId]?.value : undefined;
+
+  const textPartIsHtml = textPart?.type?.toLowerCase() === "text/html";
+  // A missing type is treated as HTML, matching the viewer's rendering path.
+  const htmlPartIsHtml = !htmlPart?.type || htmlPart.type.toLowerCase() === "text/html";
+
+  const body = textValue
+    ? (textPartIsHtml ? htmlToPlainText(textValue, { paragraphSpacing: true }) : textValue)
+    : (email.preview || "");
+  return {
+    body,
+    htmlBody: htmlPartIsHtml ? htmlValue || undefined : undefined,
+  };
+}
+
 export function plainTextToComposerBody(text: string): string {
   if (!text) return "";