diff --git a/package.json b/package.json
index 3a1684e..e6309a1 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@schemavaults/send-email",
   "description": "Schema defining the shape of the request body to send an email via @schemavaults/mail-server",
-  "version": "0.0.17",
+  "version": "0.0.18",
   "license": "UNLICENSED",
   "private": false,
   "repository": {
diff --git a/src/send-email.ts b/src/send-email.ts
index ccb470b..be19c4d 100644
--- a/src/send-email.ts
+++ b/src/send-email.ts
@@ -55,10 +55,19 @@ export async function sendEmail({
     bearerToken = getSchemaVaultsMailApiKey();
   }
 
-  const mail_server_url: string = getHardcodedApiServerDomain(
-    SCHEMAVAULTS_MAIL_APP_DEFINITION.app_id,
-    environment,
-  ).domain;
+  let mail_server_url: string;
+  if (typeof opts.mailServerUrl === "string") {
+    mail_server_url = opts.mailServerUrl;
+  } else if (typeof opts.mailServerUrl === "undefined") {
+    mail_server_url = getHardcodedApiServerDomain(
+      SCHEMAVAULTS_MAIL_APP_DEFINITION.app_id,
+      environment,
+    ).domain;
+  } else {
+    throw new TypeError(
+      "Expected 'mailServerUrl' to be a string or undefined!",
+    );
+  }
   const endpoint: string = `${mail_server_url}/api/send`;
   const response = await fetch(endpoint, {
     method: "POST",