feat: add custom email headers (#260)

apps/docs/api-reference/openapi.json

@@ -1140,6 +1140,14 @@
                     "nullable": true,
                     "minLength": 1
                   },
+                  "headers": {
+                    "type": "object",
+                    "additionalProperties": {
+                      "type": "string",
+                      "minLength": 1
+                    },
+                    "description": "Custom headers to included with the emails"
+                  },
                   "attachments": {
                     "type": "array",
                     "items": {
@@ -1288,6 +1296,14 @@
                       "nullable": true,
                       "minLength": 1
                     },
+                    "headers": {
+                      "type": "object",
+                      "additionalProperties": {
+                        "type": "string",
+                        "minLength": 1
+                      },
+                      "description": "Custom headers to included with the emails"
+                    },
                     "attachments": {
                       "type": "array",
                       "items": {

apps/docs/get-started/nodejs.mdx

@@ -57,8 +57,13 @@ icon: node-js
       subject: "useSend email",
       html: "<p>useSend is the best open source product to send emails</p>",
       text: "useSend is the best open source product to send emails",
+      headers: {
+        "X-Campaign": "welcome",
+      },
     });
     ```
+
+    > Custom headers are forwarded as-is. useSend only manages the `X-Usesend-Email-ID` and `References` headers.
   </Step>
 </Steps>
 

apps/docs/get-started/python.mdx

@@ -41,12 +41,15 @@ payload: types.EmailCreate = {
     "from": "no-reply@yourdomain.com",
     "subject": "Welcome",
     "html": "<strong>Hello!</strong>",
+    "headers": {"X-Campaign": "welcome"},
 }
 
 data, err = client.emails.send(payload)
 print(data or err)
 ```
 
+useSend forwards your custom headers to SES. Only the `X-Usesend-Email-ID` and `References` headers are managed automatically.
+
 Attachments and scheduling:
 
 ```python