From 11c428d4ffe7c8aa87aac4d24148aaa95e01507a Mon Sep 17 00:00:00 2001
From: Matthias Endler <matthias@endler.dev>
Date: Thu, 30 Apr 2026 15:20:06 +0200
Subject: [PATCH] fix(ses): emit List-Unsubscribe inside the header section,
 not the body
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The raw MIME template in sendRawEmail produced a blank line between Content-Type and List-Unsubscribe whenever no custom headers were passed, because the ternary for custom headers expanded to an empty string surrounded by newlines.

Per RFC 5322 §2.1 a blank line terminates the header section, so List-Unsubscribe ended up as the first line of the body. Lenient clients (Gmail, Outlook) recover; strict clients (Thunderbird) do not, breaking one-click unsubscribe and degrading Gmail/Yahoo bulk-sender deliverability signals.

Collect optional headers (custom + List-Unsubscribe) into an array, filter empties, and append them to the Content-Type line with a single newline separator. No codepath can now produce a blank line inside the header block.
---
 apps/api/src/services/SESService.ts | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/apps/api/src/services/SESService.ts b/apps/api/src/services/SESService.ts
index d96cd73..2a8eacd 100644
--- a/apps/api/src/services/SESService.ts
+++ b/apps/api/src/services/SESService.ts
@@ -133,21 +133,23 @@ export async function sendRawEmail({
     rootContentType = `multipart/related; boundary="${relatedBoundary}"`;
   }
 
+  // Build the additional headers (custom headers + List-Unsubscribe), filtering
+  // out empties so we never emit a blank line inside the header section.
+  // Per RFC 5322 §2.1, a blank line terminates the header section, so any blank
+  // line here would push subsequent headers (notably List-Unsubscribe) into the body.
+  const extraHeaderLines = [
+    ...(headers ? Object.entries(headers).map(([key, value]) => `${key}: ${value}`) : []),
+    ...(unsubscribeHeader ? [unsubscribeHeader] : []),
+  ];
+  const extraHeaders = extraHeaderLines.length > 0 ? `\n${extraHeaderLines.join('\n')}` : '';
+
   // Build raw MIME message
   let rawMessage = `From: ${from.name} <${from.email}>
 To: ${toHeader}
 Reply-To: ${reply || from.email}
 Subject: ${content.subject}
 MIME-Version: 1.0
-Content-Type: ${rootContentType}
-${
-  headers
-    ? Object.entries(headers)
-        .map(([key, value]) => `${key}: ${value}`)
-        .join('\n')
-    : ''
-}
-${unsubscribeHeader}
+Content-Type: ${rootContentType}${extraHeaders}
 
 `;