diff --git a/components/email/__tests__/compose-format.test.ts b/components/email/__tests__/compose-format.test.ts
index 5837505f8..b53a558c4 100644
--- a/components/email/__tests__/compose-format.test.ts
+++ b/components/email/__tests__/compose-format.test.ts
@@ -83,4 +83,10 @@ describe('compose-format (#1022): HTML <-> plain text switch', () => {
     const back = plainComposeBodyToHtml(text, identity, { separator: true });
     expect(back).toBe(start);
   });
+
+  it('adds no separator paragraph in front of a signature that has its own delimiter', () => {
+    const html = buildEmbeddedSignatureHtml({ textSignature: '-- \nUser A' }, { embed: true, separator: true });
+    expect(html).not.toContain(`${SIGNATURE_RANGE_MARKER}="separator"`);
+    expect(count(html, '-- ')).toBe(1);
+  });
 });
diff --git a/components/email/compose-format.ts b/components/email/compose-format.ts
index 4d4e918a6..0cc0a9f98 100644
--- a/components/email/compose-format.ts
+++ b/components/email/compose-format.ts
@@ -6,6 +6,7 @@ import {
   plainTextBodyHasSignature,
   plainTextBodyWithoutSignature,
 } from "@/lib/signature-utils";
+import { wantsSignatureSeparator } from "@/lib/signature-delimiter";
 import { buildSignatureBlock, SIGNATURE_RANGE_MARKER } from "@/components/email/signature-block";
 
 /**
@@ -36,7 +37,7 @@ export function buildEmbeddedSignatureHtml(
   options: { embed: boolean; separator: boolean }
 ): string {
   if (!options.embed) return '';
-  const startMarker = options.separator
+  const startMarker = wantsSignatureSeparator(options.separator, identity)
     ? `<p ${SIGNATURE_RANGE_MARKER}="separator">-- </p>`
     : `<p ${SIGNATURE_RANGE_MARKER}="start"></p>`;
   const endMarker = `<p ${SIGNATURE_RANGE_MARKER}="end"></p>`;
diff --git a/components/email/email-composer.tsx b/components/email/email-composer.tsx
index 1d6a3d68c..509b6e5bc 100644
--- a/components/email/email-composer.tsx
+++ b/components/email/email-composer.tsx
@@ -47,6 +47,7 @@ import { TemplatePicker } from "@/components/templates/template-picker";
 import { TemplateForm } from "@/components/templates/template-form";
 import type { EmailTemplate } from "@/lib/template-types";
 import { appendPlainTextSignature, getPlainTextSignature, plainTextBodyHasSignature, plainTextBodyWithoutSignature } from "@/lib/signature-utils";
+import { wantsSignatureSeparator } from "@/lib/signature-delimiter";
 import { findComposeIdentityId, findDraftIdentityId, findReplyIdentityId, resolveReplyFrom } from "@/lib/reply-identity";
 import { buildReplyRecipients, isSelfSent } from "@/lib/reply-recipients";
 import { computeReplyThreadingHeaders, type ReplyThreadingHeaders } from "@/lib/email-threading";
@@ -438,7 +439,7 @@ export function EmailComposer({
       // editable. Fixes #329 (A,B).
       if (mode === 'compose') {
         if (!shouldEmbedSignatureInNewMail) return prefix;
-        const sep = signatureSeparatorEnabled ? '\n\n-- \n' : '\n\n';
+        const sep = wantsSignatureSeparator(signatureSeparatorEnabled, initialSignatureIdentity) ? '\n\n-- \n' : '\n\n';
         return `${prefix}${sep}${getPlainTextSignature(initialSignatureIdentity)}`;
       }
       if (!replyTo?.body && !replyTo?.htmlBody) return prefix;
@@ -460,7 +461,7 @@ export function EmailComposer({
       // drafting area and the quoted content so it reads naturally as a
       // closing for the reply body. Send-time append is skipped - see
       // shouldEmbedSignatureAboveQuote.
-      const plainSep = signatureSeparatorEnabled ? '\n\n-- \n' : '\n\n';
+      const plainSep = wantsSignatureSeparator(signatureSeparatorEnabled, initialSignatureIdentity) ? '\n\n-- \n' : '\n\n';
       const signatureBlock = shouldEmbedSignatureAboveQuote
         ? `${plainSep}${getPlainTextSignature(initialSignatureIdentity)}`
         : '';
@@ -2481,7 +2482,7 @@ export function EmailComposer({
     // Build HTML signature block (used only in rich text mode)
     const buildSignatureHtml = (): string => {
       if (signatureAlreadyInBody) return '';
-      const sep = signatureSeparatorEnabled ? `<br><br>-- <br>` : `<br><br>`;
+      const sep = wantsSignatureSeparator(signatureSeparatorEnabled, signatureIdentity) ? `<br><br>-- <br>` : `<br><br>`;
       if (signatureIdentity?.htmlSignature) {
         return `${sep}${sanitizeSignatureHtml(signatureIdentity.htmlSignature)}`;
       }
@@ -3342,13 +3343,13 @@ export function EmailComposer({
           : plainTextMode ? (
           getPlainTextSignature(signatureIdentity) ? (
             <div className="px-4 pb-3 text-sm leading-6 text-muted-foreground break-words whitespace-pre-wrap font-mono">
-              {signatureSeparatorEnabled ? '-- \n' : ''}{getPlainTextSignature(signatureIdentity)}
+              {wantsSignatureSeparator(signatureSeparatorEnabled, signatureIdentity) ? '-- \n' : ''}{getPlainTextSignature(signatureIdentity)}
             </div>
           ) : null
         ) : composerSignatureHtml ? (
           <div
             className="px-4 pb-3 text-sm leading-6 text-foreground break-words [&_a]:text-primary [&_a]:underline-offset-2 [&_a:hover]:underline"
-            dangerouslySetInnerHTML={{ __html: `${signatureSeparatorEnabled ? '<div>-- </div>' : ''}${composerSignatureHtml}` }}
+            dangerouslySetInnerHTML={{ __html: `${wantsSignatureSeparator(signatureSeparatorEnabled, signatureIdentity) ? '<div>-- </div>' : ''}${composerSignatureHtml}` }}
           />
         ) : null}
       </div>
diff --git a/lib/__tests__/html-to-text.test.ts b/lib/__tests__/html-to-text.test.ts
index 4f3bcee27..8473fcdaf 100644
--- a/lib/__tests__/html-to-text.test.ts
+++ b/lib/__tests__/html-to-text.test.ts
@@ -54,4 +54,10 @@ describe('htmlToPlainText', () => {
   it('handles nested lists as separate lines', () => {
     expect(htmlToPlainText('<ul><li>One</li><li>Two</li></ul>')).toBe('One\nTwo');
   });
+
+  it('keeps the trailing space of a "-- " signature delimiter', () => {
+    // Without it the text part's delimiter was "--", which clients do not
+    // recognise as the start of a signature.
+    expect(htmlToPlainText('<p>Hi</p><p>-- </p><p>User A</p>')).toBe('Hi\n-- \nUser A');
+  });
 });
diff --git a/lib/__tests__/signature-utils.test.ts b/lib/__tests__/signature-utils.test.ts
index 795e57419..d8e717456 100644
--- a/lib/__tests__/signature-utils.test.ts
+++ b/lib/__tests__/signature-utils.test.ts
@@ -116,4 +116,23 @@ describe('signature-utils', () => {
       expect(hasMeaningfulHtmlBody('<div>Hello world</div>')).toBe(false);
     });
   });
+
+  describe('signature delimiter', () => {
+    // Identities often carry their own "-- " line; the composer's separator on
+    // top of it showed two delimiter lines.
+    const own = { textSignature: '-- \nUser A\nstw-test' };
+
+    it('keeps the space of the "-- " delimiter', () => {
+      expect(getPlainTextSignature(own)).toBe('-- \nUser A\nstw-test');
+    });
+
+    it('adds no second delimiter when the signature has its own', () => {
+      expect(appendPlainTextSignature('Hello', own, { separator: true })).toBe('Hello\n\n-- \nUser A\nstw-test');
+      expect(appendHtmlSignature('<p>Hello</p>', own, { separator: true })).toBe('<p>Hello</p><br><br>-- <br>User A<br>stw-test');
+    });
+
+    it('still adds the delimiter for a signature without one', () => {
+      expect(appendPlainTextSignature('Hello', { textSignature: 'User A' }, { separator: true })).toBe('Hello\n\n-- \nUser A');
+    });
+  });
 });
\ No newline at end of file
diff --git a/lib/html-to-text.ts b/lib/html-to-text.ts
index 941bbbcb0..64dfb9a3a 100644
--- a/lib/html-to-text.ts
+++ b/lib/html-to-text.ts
@@ -61,7 +61,11 @@ function normalizeLineBreaks(value: string): string {
   return value
     .replace(/\r\n?/g, '\n')
     .replace(new RegExp(String.fromCharCode(160), 'g'), ' ')
-    .replace(/[ \t]+\n/g, '\n')
+    // Trims trailing spaces, but keeps the one that makes "-- " the RFC 3676
+    // signature delimiter, so the text part still marks the signature.
+    .split('\n')
+    .map((line) => (/^--[ \t]*$/.test(line) ? '-- ' : line.replace(/[ \t]+$/, '')))
+    .join('\n')
     .replace(/\n{3,}/g, '\n\n')
     .trim();
 }
diff --git a/lib/signature-delimiter.ts b/lib/signature-delimiter.ts
new file mode 100644
index 000000000..76a513852
--- /dev/null
+++ b/lib/signature-delimiter.ts
@@ -0,0 +1,27 @@
+import { sanitizeSignatureHtml } from '@/lib/email-sanitization';
+import { htmlToPlainText } from '@/lib/html-to-text';
+
+type SignatureSource = {
+  textSignature?: string;
+  htmlSignature?: string;
+};
+
+/**
+ * Whether the signature starts with its own "-- " delimiter line. Many do
+ * (imported from other clients, or set on the server), and the composer's own
+ * separator on top of it showed two delimiter lines.
+ */
+export function signatureHasOwnDelimiter(signature?: SignatureSource | null): boolean {
+  const text = signature?.textSignature?.trim()
+    ? signature.textSignature
+    : signature?.htmlSignature?.trim()
+      ? htmlToPlainText(sanitizeSignatureHtml(signature.htmlSignature))
+      : '';
+  const firstLine = text.replace(/\r\n?/g, '\n').replace(/^\s*\n/, '').split('\n')[0];
+  return /^--[ \t\u00a0]*$/.test(firstLine);
+}
+
+/** Whether to add the "-- " separator line in front of this signature. */
+export function wantsSignatureSeparator(enabled: boolean | undefined, signature?: SignatureSource | null): boolean {
+  return enabled !== false && !signatureHasOwnDelimiter(signature);
+}
diff --git a/lib/signature-utils.ts b/lib/signature-utils.ts
index 180ae7854..d130c0d11 100644
--- a/lib/signature-utils.ts
+++ b/lib/signature-utils.ts
@@ -1,5 +1,6 @@
 import { parseHtmlSafely, sanitizeSignatureHtml } from '@/lib/email-sanitization';
 import { htmlToPlainText } from '@/lib/html-to-text';
+import { wantsSignatureSeparator } from '@/lib/signature-delimiter';
 
 type SignatureSource = {
   textSignature?: string;
@@ -10,7 +11,11 @@ function normalizeSignatureLineBreaks(value: string): string {
   return value
     .replace(/\r\n?/g, '\n')
     .replace(/\u00a0/g, ' ')
-    .replace(/[ \t]+\n/g, '\n')
+    // Trims trailing spaces, but keeps the one that makes "-- " the RFC 3676
+    // signature delimiter.
+    .split('\n')
+    .map((line) => (/^--[ \t]*$/.test(line) ? '-- ' : line.replace(/[ \t]+$/, '')))
+    .join('\n')
     .replace(/\n{3,}/g, '\n\n')
     .trim();
 }
@@ -37,7 +42,7 @@ export function appendPlainTextSignature(
     return body;
   }
 
-  const sep = options.separator === false ? '\n\n' : '\n\n-- \n';
+  const sep = wantsSignatureSeparator(options.separator, signature) ? '\n\n-- \n' : '\n\n';
   return `${body}${sep}${plainTextSignature}`;
 }
 
@@ -95,7 +100,7 @@ export function appendHtmlSignature(
   signature?: SignatureSource | null,
   options: { separator?: boolean } = {},
 ): string {
-  const sep = options.separator === false ? '<br><br>' : '<br><br>-- <br>';
+  const sep = wantsSignatureSeparator(options.separator, signature) ? '<br><br>-- <br>' : '<br><br>';
 
   if (signature?.htmlSignature?.trim()) {
     return `${htmlBody}${sep}${sanitizeSignatureHtml(signature.htmlSignature)}`;