diff --git a/web/src/lib/__tests__/htmlAlternative.test.ts b/web/src/lib/__tests__/htmlAlternative.test.ts
new file mode 100644
index 0000000..0fa86c6
--- /dev/null
+++ b/web/src/lib/__tests__/htmlAlternative.test.ts
@@ -0,0 +1,54 @@
+import { describe, expect, it } from "vitest";
+import { hasHtmlAlternative } from "../html";
+
+/*
+ * The rule: `htmlBody` is derived, so its presence proves nothing. Only the
+ * part's own type says whether there is an HTML alternative to render.
+ *
+ * The shapes below are what Stalwart 0.16.21 actually returned for one thread
+ * on 2026-09-10, read back through `Email/get` with
+ * `bodyProperties: ["partId", "type"]`. A plain-text message named the *same*
+ * part in both lists; a message with a real alternative named two.
+ *
+ * Getting this wrong is not a rendering nicety. Plain text went to the HTML
+ * path, which places the body under `white-space: normal`, so every line break
+ * collapsed: hard-wrapped mail arrived as one paragraph, and the signature and
+ * the quoted reply ran into the prose.
+ */
+describe("deciding whether a message has an HTML alternative", () => {
+  it("says no to a plain-text message, whose htmlBody holds the text part", () => {
+    // As returned for mo@bunkus.online: htmlBody[0] and textBody[0] are the
+    // same part, typed text/plain.
+    expect(hasHtmlAlternative({ type: "text/plain" }, "Hey,\n\nmy earlier response was before\n")).toBe(false);
+  });
+
+  it("says yes to a real multipart/alternative", () => {
+    expect(hasHtmlAlternative({ type: "text/html" }, "<p>Hello</p>")).toBe(true);
+  });
+
+  it("keeps the parameters that follow a media type", () => {
+    // `type` arrives bare in practice, but a charset must not turn a real HTML
+    // part into a plain-text one.
+    expect(hasHtmlAlternative({ type: "text/html; charset=utf-8" }, "<p>Hi</p>")).toBe(true);
+  });
+
+  it("is not fooled by a type that merely starts with the right letters", () => {
+    expect(hasHtmlAlternative({ type: "text/htmlish" }, "<p>Hi</p>")).toBe(false);
+  });
+
+  it("matches the type case-insensitively, since a header may be capitalised", () => {
+    expect(hasHtmlAlternative({ type: "TEXT/HTML" }, "<p>Hi</p>")).toBe(true);
+  });
+
+  it("says no when the part is HTML but its value never arrived", () => {
+    // maxBodyValueBytes can leave a part named with nothing fetched; falling
+    // through to the text body is the useful answer, not an empty pane.
+    expect(hasHtmlAlternative({ type: "text/html" }, undefined)).toBe(false);
+    expect(hasHtmlAlternative({ type: "text/html" }, "")).toBe(false);
+  });
+
+  it("says no when there is no part at all", () => {
+    expect(hasHtmlAlternative(undefined, undefined)).toBe(false);
+    expect(hasHtmlAlternative({}, "something")).toBe(false);
+  });
+});
diff --git a/web/src/lib/html.ts b/web/src/lib/html.ts
index 23a9d51..88ff172 100644
--- a/web/src/lib/html.ts
+++ b/web/src/lib/html.ts
@@ -350,6 +350,26 @@ export function markKeptSurfaces(root: ParentNode): number {
   return kept;
 }
 
+/**
+ * Whether a message really has an HTML alternative to render.
+ *
+ * `htmlBody` is a *derived* list, not a filter: RFC 8621 §4.1.4 says a message
+ * with no HTML alternative still gets one, and it holds the text/plain part.
+ * Confirmed live against Stalwart 0.16.21 (2026-09-10) -- a plain-text mail
+ * comes back with `htmlBody` and `textBody` naming the same part, typed
+ * `text/plain`, while a real multipart/alternative names two different parts.
+ *
+ * So "is there a body value under htmlBody" is not the question; the part's own
+ * type is. Answering the first one sent every plain-text message down the HTML
+ * path, where the body is placed in `.ihm-email-root` under
+ * `white-space: normal` and every line break collapses -- hard-wrapped mail
+ * arrived as a single paragraph with the signature and the quoted reply run
+ * into the prose.
+ */
+export function hasHtmlAlternative(part: { type?: string } | undefined, value: string | undefined): boolean {
+  return /^text\/html\b/i.test(part?.type ?? "") && Boolean(value);
+}
+
 export const TEXT_EMAIL_CSS = `
 :host { display:block; }
 .ihm-text-root { font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, "Liberation Mono", monospace; font-size: 13.5px; line-height:1.55; white-space: pre-wrap; overflow-wrap: anywhere; color: inherit; }
diff --git a/web/src/views/mail/MessageView.tsx b/web/src/views/mail/MessageView.tsx
index 2840059..0beec3d 100644
--- a/web/src/views/mail/MessageView.tsx
+++ b/web/src/views/mail/MessageView.tsx
@@ -18,7 +18,7 @@ import { internalDomains, isExternalSender, linkVerdict } from "@/lib/warnings";
 import { spamReport, type SpamReport } from "@/lib/spamScore";
 import { formatFullDate, formatListDate, formatSize } from "@/lib/format";
 import { displayName, domainOf, formatAddress } from "@/lib/address";
-import { EMAIL_BASE_CSS, TEXT_EMAIL_CSS, htmlDeclaresColors, markKeptSurfaces, sanitizeEmailHtml } from "@/lib/html";
+import { EMAIL_BASE_CSS, TEXT_EMAIL_CSS, hasHtmlAlternative, htmlDeclaresColors, markKeptSurfaces, sanitizeEmailHtml } from "@/lib/html";
 import { openableInTab, previewKind } from "@/lib/preview";
 import { FilePreviewDialog } from "@/ui/filepreview";
 import { findQuoteStart, htmlToText, textToHtml } from "@/lib/text";
@@ -137,7 +137,9 @@ export const MessageView = memo(function MessageView({ email: e, expanded, wasUn
   const textPart = e.textBody?.[0];
   const htmlRaw = htmlPart?.partId ? e.bodyValues?.[htmlPart.partId]?.value : undefined;
   const textRaw = textPart?.partId ? e.bodyValues?.[textPart.partId]?.value : undefined;
-  const showHtml = Boolean(htmlRaw);
+  // Not `Boolean(htmlRaw)`: `htmlBody` carries the text part when there is no
+  // HTML alternative. See hasHtmlAlternative().
+  const showHtml = hasHtmlAlternative(htmlPart, htmlRaw);
   const themeMessageBody = settings.themeMessageBody;
   const themeStyledMessages = settings.themeStyledMessages;