diff --git a/components/email/__tests__/composer-recipient-validation.test.tsx b/components/email/__tests__/composer-recipient-validation.test.tsx
index de2414ee6..5f250a716 100644
--- a/components/email/__tests__/composer-recipient-validation.test.tsx
+++ b/components/email/__tests__/composer-recipient-validation.test.tsx
@@ -256,6 +256,19 @@ describe('composer recipient validation', () => {
     expect(toastError).toHaveBeenCalledWith('validation.invalid_recipients');
   });
 
+  it('does not send a group whose member is not an address and names the member', async () => {
+    const onSend = vi.fn();
+    render(<EmailComposer initialData={{ ...DRAFT, to: 'Team: bob@example.com, junk;' }} onSend={onSend} />);
+
+    expect(screen.getByText('Team (2)').closest('[data-invalid]')).not.toBeNull();
+
+    fireEvent.click(sendButton());
+    await act(async () => {});
+
+    expect(onSend).not.toHaveBeenCalled();
+    expect(toastError).toHaveBeenCalledWith('validation.invalid_recipients');
+  });
+
   it('sends when every recipient is an address', async () => {
     const onSend = vi.fn();
     render(<EmailComposer initialData={{ ...DRAFT, to: 'bob@example.com' }} onSend={onSend} />);
diff --git a/components/email/email-composer.tsx b/components/email/email-composer.tsx
index afda215ab..6983aa4b6 100644
--- a/components/email/email-composer.tsx
+++ b/components/email/email-composer.tsx
@@ -105,6 +105,7 @@ import {
   formatRecipientList,
   expandRecipients,
   splitPastedRecipients,
+  invalidRecipientAddresses,
   defaultSuggestionIndex,
   waitForPendingUploads,
   extractUserAuthoredText,
@@ -2881,11 +2882,11 @@ export function EmailComposer({
 
     // Catch addresses the server would reject before asking it. Its answer
     // ("No recipients found in email") does not say which one is wrong.
-    const invalidRecipients = [
+    const invalidRecipients = invalidRecipientAddresses([
       ...withInput(to, toInput),
       ...withInput(cc, ccInput),
       ...withInput(bcc, bccInput),
-    ].filter(r => !r.group && !isValidEmail(r.email)).map(r => r.email);
+    ]);
     if (invalidRecipients.length > 0) {
       toast.error(t('validation.invalid_recipients', { addresses: invalidRecipients.join(', ') }));
       return;
@@ -4999,7 +5000,7 @@ function RecipientChipInput({
           if(chip.extra?.icon){
              IconComponent = ICON_MAP[chip.extra?.icon];
           }
-          const isInvalid = !chip.group && !isValidEmail(chip.email);
+          const isInvalid = invalidRecipientAddresses([chip]).length > 0;
           const customColor = isInvalid ? 'destructive' : chip.extra?.color;
 
           return (
diff --git a/lib/__tests__/email-composer-utils.test.ts b/lib/__tests__/email-composer-utils.test.ts
index 117614a79..c835f0963 100644
--- a/lib/__tests__/email-composer-utils.test.ts
+++ b/lib/__tests__/email-composer-utils.test.ts
@@ -16,6 +16,7 @@ import {
   extractUserAuthoredText,
   formatRecipientEntry,
   expandRecipients,
+  invalidRecipientAddresses,
   getQuoteBodies,
 } from '../email-composer-utils';
 
@@ -594,6 +595,16 @@ describe('contact group recipients (RFC 5322 group syntax)', () => {
     expect(parsed[1].group?.members).toHaveLength(2);
   });
 
+  it('keeps typed text with a colon as a plain recipient when no member is an address', () => {
+    expect(parseRecipient('Subject: hello;')).toEqual({ email: 'Subject: hello;' });
+  });
+
+  it('lists group members that are not addresses as invalid', () => {
+    const parsed = parseRecipientList('ok@x.de, Team: a@x.de, junk;, nope');
+    expect(invalidRecipientAddresses(parsed)).toEqual(['junk', 'nope']);
+    expect(invalidRecipientAddresses([group])).toEqual([]);
+  });
+
   it('expandRecipients flattens groups and dedupes against individuals', () => {
     const expanded = expandRecipients([
       { name: 'Anna Alt', email: 'ANNA@example.com' },
diff --git a/lib/email-composer-utils.ts b/lib/email-composer-utils.ts
index a3cc1c1b6..0ac4babef 100644
--- a/lib/email-composer-utils.ts
+++ b/lib/email-composer-utils.ts
@@ -383,9 +383,9 @@ export function parseRecipient(s: string): Recipient {
       const members = splitRecipients(trimmed.slice(colon + 1, -1))
         .map(parseRecipient)
         .filter((m) => m.email && !m.group);
-      // Only accept the group form when it actually carries members - typed
-      // garbage like "Subject: hello;" stays a plain (invalid) recipient.
-      if (members.length > 0) {
+      // Only accept the group form when at least one member is an address -
+      // typed garbage like "Subject: hello;" stays a plain (invalid) recipient.
+      if (members.some((m) => isValidEmail(m.email))) {
         return { name: unquoteName(trimmed.slice(0, colon)), email: '', group: { members } };
       }
     }
@@ -462,6 +462,18 @@ export function expandRecipients(recipients: Recipient[]): Recipient[] {
   return out;
 }
 
+/**
+ * The entries of `recipients` that are not email addresses, group members
+ * included. Sending any of them puts a malformed mailbox into the header,
+ * which relays reject only after the whole message went through (#795).
+ */
+export function invalidRecipientAddresses(recipients: Recipient[]): string[] {
+  return recipients
+    .flatMap((r) => (r.group ? r.group.members : [r]))
+    .filter((r) => !isValidEmail(r.email))
+    .map((r) => r.email);
+}
+
 /**
  * Top-level split of a pasted block into recipient entries on commas,
  * semicolons and newlines (separators inside a quoted name or angle brackets