diff --git a/apps/api/src/services/ContactService.ts b/apps/api/src/services/ContactService.ts
index 35ae603..e3b792d 100644
--- a/apps/api/src/services/ContactService.ts
+++ b/apps/api/src/services/ContactService.ts
@@ -225,12 +225,14 @@ export class ContactService {
     // Merge new data (if provided)
     if (data) {
       for (const [key, value] of Object.entries(data)) {
-        // Skip reserved fields
-        if (key === 'plunk_id' || key === 'plunk_email') {
+        // Skip reserved system-generated fields
+        // These fields are dynamically added during template rendering and cannot be overridden
+        const reservedFields = ['plunk_id', 'plunk_email', 'email', 'unsubscribeUrl', 'subscribeUrl', 'manageUrl'];
+        if (reservedFields.includes(key)) {
           continue;
         }
 
-        // Validate locale field
+        // Validate locale field (special user-settable field)
         if (key === 'locale') {
           if (typeof value === 'string') {
             if (!isValidLanguageCode(value)) {
@@ -321,8 +323,9 @@ export class ContactService {
     // Add temporary (non-persistent) data
     if (temporaryData) {
       for (const [key, value] of Object.entries(temporaryData)) {
-        // Skip reserved fields
-        if (key === 'plunk_id' || key === 'plunk_email') {
+        // Skip reserved system-generated fields
+        const reservedFields = ['plunk_id', 'plunk_email', 'email', 'unsubscribeUrl', 'subscribeUrl', 'manageUrl'];
+        if (reservedFields.includes(key)) {
           continue;
         }
 
diff --git a/apps/wiki/content/docs/concepts/contacts-and-data.mdx b/apps/wiki/content/docs/concepts/contacts-and-data.mdx
index c6cbcae..11bb17d 100644
--- a/apps/wiki/content/docs/concepts/contacts-and-data.mdx
+++ b/apps/wiki/content/docs/concepts/contacts-and-data.mdx
@@ -85,7 +85,14 @@ Use `{{fieldName}}` in emails:
 
 **Fallback:** `{{firstName ?? 'there'}}`
 
-**Predefined fields:** `{{email}}`, `{{unsubscribeUrl}}`, `{{subscribeUrl}}`, `{{manageUrl}}`, `{{locale}}`
+**System-generated fields (always available):**
+- `{{email}}` - Contact's email address
+- `{{unsubscribeUrl}}` - Link to unsubscribe page
+- `{{subscribeUrl}}` - Link to subscribe page
+- `{{manageUrl}}` - Link to preference center
+
+**Special fields:**
+- `{{locale}}` - Contact's language preference (user-settable, overrides project default)
 
 ## Temporary data