If you upload a file to Letterbatch, where does it go?

The short answer is that it stays on your device.

I realize that sounds strange. You are using a website. You click a button that says upload. You select a spreadsheet full of names, email addresses, or other information. How can Letterbatch say it never received the file?

That question is more useful than a vague promise that your data is safe, so here is how it actually works.

Selecting a file is not always the same as sending it

When you select a file in Letterbatch, your browser gives the application permission to read that specific file on your device.

The Letterbatch code running in your browser opens the file there. It reads the spreadsheet, applies the matching rules you choose, prepares the documents, and builds the finished outputs using your device. The recipient rows and document contents are not sent to a Letterbatch server for processing.

The process looks like this:

1. You select a file from your device.
2. Your browser reads it locally.
3. Letterbatch processes and combines the information inside that browser session.
4. The finished batch is generated on your device and downloaded back to you.

If you save your work, Letterbatch creates a batch file on your device. It also keeps a recovery checkpoint in that browser so an interrupted session is less likely to erase your progress. That checkpoint is specific to that browser and device. It is not a cloud backup.

This is closer to opening a file in an application on your computer than sending a file to a typical cloud service. Letterbatch happens to run in a browser tab instead of arriving as installed desktop software.

What still uses the internet

Letterbatch is not an offline desktop application, and I do not want to imply that it makes no network requests.

The internet delivers the application to your browser. Letterbatch also uses network services to sign you in, operate your account, and send limited anonymous product-health counts. Those requests do not contain recipient names, file contents, filenames, column names, email content, or generated documents.

Letterbatch also does not require an Outlook or Office add-in. It does not receive access to your mailbox or Microsoft 365 tenant, and no AI model receives or processes the communication data you use in the product. You review the finished work and move it into the email or mail systems your organization already controls.

That is the boundary: the application uses the internet. Your communication data is processed on your device.

Why I built it this way

There is no shortage of software for specific workplace problems. The devastating part is finding the tool that solves exactly what you need, then realizing that using it would require uploading sensitive files, connecting your Microsoft 365 account, or installing something your organization cannot approve.

I understand why those reviews exist. They protect the organization and the information people have trusted it to hold. I did not build Letterbatch to help people work around IT or procurement. I built it so Letterbatch would have less to ask for in the first place.

The local-first software principles described by Ink & Switch gave me language for that choice. Their complete model is broader than what Letterbatch currently claims. The principle I adopted is straightforward: the user’s copy of the work should remain with the user.

Local processing does not remove responsibility. A saved Letterbatch batch contains the customer data you brought into the product and should be protected like the source files. A compromised device is still a compromised device. Your organization’s browser, file, data-handling, and Power Automate policies still apply, and your organization may still need to review Letterbatch.

The point is not to escape the security conversation. It is to make that conversation smaller and more specific.

I have been the person who found a useful tool and knew how difficult it would be to get permission to connect it to the systems or data involved. I wanted Letterbatch to meet people inside the real constraints of their working environment.

If this is the kind of workflow you are carrying, request an invitation to Letterbatch and tell me what you are preparing.