Rendering errors

Lob occasionally encounters problems with the creative when rendering mailpieces; this can occur with either PDFs or HTML. To avoid sending out erroneous mailpieces, Lob provides visibility of these errors, with proactive notification of rendering errors in place.

We always recommend testing; and where possible, sending yourself a printed mailpiece.


Lob will fail incorrectly formatted PDFs. See our PDF preflight checklist here to ensure your file will process through our rendering engine.


Lob will fail HTML that includes inaccessible assets (images, fonts, any other URLs):

  • the link to an asset is incorrect

  • the asset referenced in the file is missing

  • Lob has not been given the correct permission to access the asset (ensure that the link is set to public viewing, and not private)

  • when a remote asset references a local asset (for example, a remote asset is at which then references something local in that directory)

  • slow or under-resourced server providing the asset (we recommend using a performant service, such as Amazon S3, to host the content)

HTML Template Preview

If your HTML contains an inaccessible asset, your HTML preview will error. Details of the error will be displayed in a future release.

In the Campaigns UI flow

  • When creating your Campaign, you may be notified when you generate a Creative Proof (if the sample mailpiece that is rendered results in an error); if this occurs you should troubleshoot against the above list.

  • After placing the order, you will be notified in the dashboard, and the reason for the error will be provided (click to enlarge):

  • The Lob support team will proactively reach out.

API users

  • API customers will receive an error notification in the API response.

  • The Lob support team will proactively reach out.


Webhooks users can subscribe to receive notifications of failed orders. (Learn more about using webhooks here.)

  1. Under Webhooks in the dashboard, in the Live environment, click Create.

  2. Check [formfactor].failed; click Create.

If subscribed to postcard.failed, letter.failed, and/or check.failed, and Lob fails an order during rendering, the following error messages are possible:

"Rendering failed due to asset issue:

  • The requested asset was not found: <asset link>"

  • The requested asset is not permitted: <asset link>"

  • Authentication is required to access the requested asset: <asset link>"

  • An internal server error occurred for: <asset link>"

  • The requested asset timed out: <asset link>"

  • An unknown server error occurred for: <asset link>"

  • The requested service is unavailable for: <asset link>"

  • An unknown error occurred for: <asset link>"

If not one of the most common errors above, or a PDF failure, "try re-render” will be returned.

If you are unable to resolve the error, please reach out to

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