Royal Mail API Error E1356: "Unknown API Error" & Tracking Ranges Explained

There is nothing more frustrating for an e-commerce manager than a sudden halt in fulfillment. Your warehouse team is packing orders, your Carrier Integration is set up, but instead of a shipping label, the system throws back a cryptic error message.

If you are shipping with Royal Mail and have suddenly encountered API Error E1356, you have hit a common, yet poorly documented roadblock. Royal Mail’s official documentation often labels E1356 simply as an "Unknown API Error," which offers zero help when you have 500 orders waiting to leave the dock.

In our experience processing millions of shipments, code E1356 almost always points to one specific issue: your tracking number allowance has run out.

In this guide, we will troubleshoot why this happens and give you the exact script to use with Royal Mail support to get your labels printing again.

The Diagnosis (Error Matrix)

Before you waste hours checking your API keys or firewall settings, check this matrix. This table is designed to help you quickly identify if E1356 is truly your problem.

Unlike other carriers that generate random numbers indefinitely, Royal Mail assigns a finite "block" of barcodes to your account (e.g., GB0001 to GB5000). When you hit GB5000, the system stops.

Why Did This Happen Now?

It is easy to overlook tracking ranges until they become a critical failure. Here are the three most common reasons why e-commerce businesses suddenly hit Error E1356:

1. The "Peak Season" Surprise: You forecasted 1,000 shipments a month, but during Black Friday Strategy, you shipped 3,000. You burned through your annual allocation in three months.

2. New Account Limits: New OBA accounts are often given smaller "probationary" ranges (e.g., only 1,000 numbers) to start.

Expired Ranges: Tracking ranges have an expiry date. If your automated renewal failed on the Royal Mail backend, your valid range disappears overnight.

The Solution (How to Fix E1356)

Unfortunately, this is not an error you can resolve inside your ShippyPro dashboard. Because the issue lies with your account’s allocation on Royal Mail’s servers, you must contact the carrier directly.

Step-by-Step Resolution:

1. Call the Right Team: Do not call general customer service. Ask specifically for the "OBA (Online Business Account) Technical Support" or the "Interpreters Team."

2. Use This Script:

   "I am receiving API Error E1356 via my shipping integrator. I believe my allocated barcode range (tracking number range) has been exhausted. Please check my current allocation and add a new block of tracking numbers to my account immediately."

3. Wait for Propagation: Once they add the range, it can take anywhere from 1 to 4 hours to update across their network.

4. Retry Printing: You do not need to change settings in ShippyPro. Once the range is active, simply select the failed orders and click "Ship" again.

FAQ: Frequently Asked Questions

How long does it take to fix Error E1356?

Once you reach the OBA technical team, adding a range takes minutes. However, the system update can take up to 24 hours in rare cases. We recommend marking urgent orders as "shipped" and using a manual backup carrier if the delay persists.

Does this error affect non-tracked services (Standard 1st Class)?

Usually, no. Error E1356 specifically impacts services that require a unique 2D barcode generation, such as Royal Mail Tracked 24/48 and Special Delivery. Standard untracked letters often use a static barcode or no barcode.

Why doesn't the error just say "Out of Tracking Numbers"?

It is a legacy issue. The Royal Mail API architecture often returns a generic "catch-all" code (E1356) for backend provisioning failures. This is why having a shipping partner like ShippyPro is vital—we decode these errors for you.