diff --git a/components/email/__tests__/composer-a11y.test.tsx b/components/email/__tests__/composer-a11y.test.tsx
index a873bcb22..f662030ca 100644
--- a/components/email/__tests__/composer-a11y.test.tsx
+++ b/components/email/__tests__/composer-a11y.test.tsx
@@ -444,3 +444,30 @@ describe('confirm before sending (MO-16)', () => {
     expect(onSend).not.toHaveBeenCalled();
   });
 });
+
+describe('default compose font (CO-16)', () => {
+  const READY = { ...DRAFT, to: 'bob@example.com' };
+
+  it('starts the editor in the chosen font and sends it with the mail', async () => {
+    settings.composeFontFamily = 'serif';
+    settings.composeFontSize = '18px';
+    const onSend = vi.fn();
+    render(<EmailComposer initialData={READY} onSend={onSend} />);
+    expect(JSON.parse(screen.getByTestId('rich-text-editor').getAttribute('data-default-style')!))
+      .toEqual({ fontFamily: 'serif', fontSize: '18px' });
+
+    fireEvent.click(screen.getAllByTestId('composer-send')[0]);
+    await waitFor(() => expect(onSend).toHaveBeenCalledTimes(1));
+    const sent = onSend.mock.calls[0][0] as { htmlBody?: string };
+    expect(sent.htmlBody).toMatch(/^<div style="font-family: serif; font-size: 18px">/);
+  });
+
+  it('leaves the mail as it was without the setting, and ignores values that are not a font', async () => {
+    settings.composeFontFamily = 'serif"><script>';
+    const onSend = vi.fn();
+    render(<EmailComposer initialData={READY} onSend={onSend} />);
+    fireEvent.click(screen.getAllByTestId('composer-send')[0]);
+    await waitFor(() => expect(onSend).toHaveBeenCalledTimes(1));
+    expect((onSend.mock.calls[0][0] as { htmlBody?: string }).htmlBody).toMatch(/^<div>/);
+  });
+});
diff --git a/components/email/__tests__/rich-text-editor-a11y.test.tsx b/components/email/__tests__/rich-text-editor-a11y.test.tsx
index ad0ddfbb9..4660c27b2 100644
--- a/components/email/__tests__/rich-text-editor-a11y.test.tsx
+++ b/components/email/__tests__/rich-text-editor-a11y.test.tsx
@@ -126,3 +126,11 @@ describe('RichTextEditor formatting toolbar (SR-08)', () => {
     expect(document.activeElement).toBe(item('font-size'));
   });
 });
+
+describe('RichTextEditor default text style (CO-16)', () => {
+  it('shows the text in the chosen font and size', async () => {
+    const { editor } = await setup({ defaultTextStyle: { fontFamily: 'monospace', fontSize: '20px' } });
+    expect(editor.view.dom.style.fontFamily).toBe('monospace');
+    expect(editor.view.dom.style.fontSize).toBe('20px');
+  });
+});
diff --git a/components/email/email-composer.tsx b/components/email/email-composer.tsx
index f09146e25..20ef42ac3 100644
--- a/components/email/email-composer.tsx
+++ b/components/email/email-composer.tsx
@@ -138,6 +138,7 @@ import {
 import type { ComposePrefill } from "@/lib/plugin-compose-gate";
 import { composerContextMode, composerMounted, composerUnmounted, draftSaved } from "@/lib/plugin-composer-hooks";
 import { isValidEmail } from "@/lib/validation";
+import { composeTextStyle, composeTextStyleCss } from "@/components/email/font-size";
 import { buildMentionCandidates, type MentionCandidate } from "@/lib/recipient-mentions";
 import { describeRecipientChanges, recipientDisplayName, type RecipientChange, type RecipientFields } from "@/lib/recipient-announcements";
 import { normalizeCid } from "@/lib/attachment-visibility";
@@ -496,6 +497,11 @@ export function EmailComposer({
   const placeholderWarningEnabled = useSettingsStore((state) => state.placeholderWarningEnabled);
   // "Confirm before sending" (an admin can lock it; the store applies the lock).
   const sendConfirmation = useSettingsStore((state) => state.sendConfirmation);
+  // The font and size messages start in (Settings > Accessibility); the
+  // sent HTML carries them, so recipients see them too.
+  const composeFontFamily = useSettingsStore((state) => state.composeFontFamily);
+  const composeFontSize = useSettingsStore((state) => state.composeFontSize);
+  const composeStyle = useMemo(() => composeTextStyle(composeFontFamily, composeFontSize), [composeFontFamily, composeFontSize]);
   const recipientMentionsEnabled = useSettingsStore((state) => state.recipientMentionsEnabled);
   const mentionAddsRecipientTo = useSettingsStore((state) => state.mentionAddsRecipientTo);
   const updateSetting = useSettingsStore((state) => state.updateSetting);
@@ -3475,11 +3481,12 @@ export function EmailComposer({
     }
 
     const rewritten = plainTextMode ? null : rewriteInlineImages(body);
+    const composeStyleCss = composeTextStyleCss(composeStyle);
     // What the send-pipeline finalisers add (organisation signature) goes in
     // here, on the send only: drafts never get it. The `last` stage (the
     // disclaimer) runs on what goes out once plugins had their say, below.
     const { html: finalHtmlBody, text: finalBody } = finalizeOutgoing({
-      html: plainTextMode ? undefined : `<div>${rewritten!.html}</div>${buildSignatureHtml()}`,
+      html: plainTextMode ? undefined : `${composeStyleCss ? `<div style="${composeStyleCss}">` : '<div>'}${rewritten!.html}</div>${buildSignatureHtml()}`,
       text: builtTextBody,
     }, outgoingContext);
     const lastStage = (html: string | undefined, text: string) =>
@@ -4453,6 +4460,7 @@ export function EmailComposer({
               ariaLabel={t('body_label')}
               ariaInvalid={validationErrors.body}
               ariaDescribedBy={`${fieldIdBase}-body-error`}
+              defaultTextStyle={composeStyle}
               collapseToolbarOnMobile={!showFormattingOnMobile}
               toolbarPlacement={mvWindow ? (mvShowFormatting ? 'bottom' : 'hidden') : 'top'}
               onEditorReady={handleEditorReady}
diff --git a/components/email/font-size.ts b/components/email/font-size.ts
index e733d9445..3e44ea381 100644
--- a/components/email/font-size.ts
+++ b/components/email/font-size.ts
@@ -77,3 +77,32 @@ declare module "@tiptap/core" {
     };
   }
 }
+
+/** The font and size a message starts with (Settings > Accessibility). */
+export interface ComposeTextStyle {
+  fontFamily?: string;
+  fontSize?: string;
+}
+
+// The settings sync from the server: only plain family lists and lengths
+// go into a style attribute.
+const SAFE_FONT_FAMILY = /^[a-z0-9 ,'-]{1,200}$/i;
+const SAFE_FONT_SIZE = /^\d{1,3}(\.\d{1,2})?(px|pt|em|rem|%)$/;
+
+/** The composeFontFamily / composeFontSize settings, checked; empty means the default. */
+export function composeTextStyle(fontFamily: string | undefined, fontSize: string | undefined): ComposeTextStyle {
+  const style: ComposeTextStyle = {};
+  const family = fontFamily?.trim();
+  const size = fontSize?.trim();
+  if (family && SAFE_FONT_FAMILY.test(family)) style.fontFamily = family;
+  if (size && SAFE_FONT_SIZE.test(size)) style.fontSize = size;
+  return style;
+}
+
+/** The style as CSS declarations for an inline style attribute ("" when there is none). */
+export function composeTextStyleCss(style: ComposeTextStyle): string {
+  return [
+    style.fontFamily ? `font-family: ${style.fontFamily}` : '',
+    style.fontSize ? `font-size: ${style.fontSize}` : '',
+  ].filter(Boolean).join('; ');
+}
diff --git a/components/email/rich-text-editor.tsx b/components/email/rich-text-editor.tsx
index 07fdfecdb..681b665af 100644
--- a/components/email/rich-text-editor.tsx
+++ b/components/email/rich-text-editor.tsx
@@ -15,7 +15,7 @@ import { PlainTextPaste } from "@/components/email/plain-text-paste";
 import { pastedImageFiles } from "@/components/email/paste-images";
 import { TextStyle, BackgroundColor } from "@tiptap/extension-text-style";
 import Color from "@tiptap/extension-color";
-import { FontSize, FONT_SIZES } from "@/components/email/font-size";
+import { FontSize, FONT_SIZES, composeTextStyleCss, type ComposeTextStyle } from "@/components/email/font-size";
 import { ResizableImage } from "@/components/email/resizable-image";
 import Placeholder from "@tiptap/extension-placeholder";
 import { Table } from "@tiptap/extension-table";
@@ -132,12 +132,16 @@ interface RichTextEditorProps {
   ariaInvalid?: boolean;
   /** The id of the text that explains the error. */
   ariaDescribedBy?: string;
+  /** The font and size the text starts in (the sent mail carries them too). */
+  defaultTextStyle?: ComposeTextStyle;
 }
 
 /** What the editing area tells assistive technology, besides TipTap's role=textbox. */
-export function editorA11yAttributes({ ariaLabel, ariaInvalid, ariaDescribedBy }: Pick<RichTextEditorProps, 'ariaLabel' | 'ariaInvalid' | 'ariaDescribedBy'>): Record<string, string> {
+export function editorA11yAttributes({ ariaLabel, ariaInvalid, ariaDescribedBy, defaultTextStyle }: Pick<RichTextEditorProps, 'ariaLabel' | 'ariaInvalid' | 'ariaDescribedBy' | 'defaultTextStyle'>): Record<string, string> {
+  const style = defaultTextStyle ? composeTextStyleCss(defaultTextStyle) : "";
   return {
     class: "tiptap min-h-[100px] px-4 py-3 text-sm text-foreground",
+    ...(style ? { style } : {}),
     role: "textbox",
     "aria-multiline": "true",
     ...(ariaLabel ? { "aria-label": ariaLabel } : {}),
@@ -362,6 +366,7 @@ export function RichTextEditor({
   ariaLabel,
   ariaInvalid,
   ariaDescribedBy,
+  defaultTextStyle,
 }: RichTextEditorProps) {
   const rtlEditingSupport = useSettingsStore((st) => st.rtlEditingSupport);
   const tComposer = useTranslations("email_composer");
@@ -470,7 +475,7 @@ export function RichTextEditor({
     },
     content,
     editorProps: {
-      attributes: editorA11yAttributes({ ariaLabel, ariaInvalid, ariaDescribedBy }),
+      attributes: editorA11yAttributes({ ariaLabel, ariaInvalid, ariaDescribedBy, defaultTextStyle }),
       handleKeyDown: (_view, event) => {
         if (event.altKey && event.key === "F10" && focusToolbarRef.current?.()) {
           event.preventDefault();
@@ -550,10 +555,10 @@ export function RichTextEditor({
     editor.setOptions({
       editorProps: {
         ...editor.options.editorProps,
-        attributes: editorA11yAttributes({ ariaLabel, ariaInvalid, ariaDescribedBy }),
+        attributes: editorA11yAttributes({ ariaLabel, ariaInvalid, ariaDescribedBy, defaultTextStyle }),
       },
     });
-  }, [editor, ariaLabel, ariaInvalid, ariaDescribedBy]);
+  }, [editor, ariaLabel, ariaInvalid, ariaDescribedBy, defaultTextStyle]);
 
   // Expose the editor instance once it's ready so parents can target
   // specific nodes (e.g. swap the embedded signature on identity change).