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Letters overview

To get started with sending letters, check out our Letters API documentation, GitHub library, or Template Gallery for inspiration.

Dimensions & specs

Lob offers 8.5" x 11" letters in both black & white and in color, which can be printed single or double-sided. Letters and checks not exceeding 6 tri-folded pages in total will be sent in standard #10 double-windowed outer envelopes. Letters and checks over 6 pages but not exceeding 60 total pages will be mailed in a flat envelope.
Reference our design templates on where to place your design elements for letters.
We use a few pre-approved papers across our commercial printer network. In ensuring uniform quality and consistency across all of our mail pieces, each paper source specification must fall within a small range. The specification/value range pairs are:
  • 8.5" x 11" B&W & color letters
    • Basis Weight: 60# text
    • GSM: 89 - 90
    • C-fold letters

Address placement

The address_placement parameter specifies the location of the address information that will show through the double-window envelope. Options are top_first_page and insert_blank_page.
By default, Lob selects address_placement = top_first_page, meaning Lob will print address information at the top of your provided first page. Make sure to leave ample space for the address information to be printed, which will show through our standard double-windowed envelope. To see how this will impact your letter design, view our letter template (6 pages or less).
If you pass address_placement = insert_blank_page, a blank address page will be inserted at the beginning of your file where we will print the address information to show through our standard double-windowed envelope.
If a blank address page is inserted, note that you will be charged for the extra page. Reference the Letter Unit Pricing section for charges you will incur based on your tier.
If you do not follow these address guidelines, your letter will be printed incorrectly.

Logo placement

Your logo should be entirely within the blue box of the letter template, with no overlap of the red box, in order to print appropriately.
If you're looking to print a color logo then ensure you're setting the color parameter to 'true' if you would like to print in color otherwise you can set it to false.

Mailing letters

Ensure your letters get to their final destination by following these guidelines:
  • The from field is required for all letters, regardless of the destination
  • Letters may contain PII as envelopes have an interior security tint for privacy
  • Mailing class options may differ based on the destination and use case (e.g. promotional vs transactional)
  • You may add QR codes to your letters to incorporate personalized tracking.

Using Certified Mail or Registered Mail

For self-service plan users, Extra Services (Certified Mail, Certified Mail with Electronic Return Receipts, and Registered Mail) will require an approval process for usage.

Certified Mail

Certified Mail is an add-on service offered by USPS for First Class mail that provides proof of mailing to the sender. With electronic USPS tracking, the sender is notified when the mailing was delivered or that a delivery attempt was made. An electronic return receipt showing the recipient's signature is also available for Certified Mail only. Both the Certified Mail and the electronic return receipt add-ons are only available for First Class mail sent domestically within the US.
You send letters as Certified Mail through the Lob API by passing the value certified in the extra_services parameter of the create letter request. You may similarly opt for an electronic return receipt by passing certified_return_receipt in the extra_services parameter in the API.
Tracking events are viewable directly within the Lob dashboard or via the API, similar to how they are for non-Certified letters, and you can also subscribe to webhook events for Certified letters. When you make a request for Certified Mail you will immediately receive a carrier tracking number (retrievable via your Lob dashboard), which can be used to track the mail via the carrier’s website - or you can choose to track the mail via the scan events within your Lob dashboard. The resulting screen should appear in your dashboard:
Tracking events for Certified Mail are different from tracking events received for regular mail types; see the full list in our API documentation for Certified tracking event details.
Visit the Certified Mail envelopes section for more guidance around sending Certified Mail.

Electronic Return Receipts

An Electronic Return Receipt is available to the sender as a USPS add-on to Certified Mail. This receipt will allow you to download a digital copy of the recipient's signature used to sign for the delivery of the letter. You opt for an Electronic Return Receipt by passing certified_return_receipt in the extra_service parameter of creating a letter.
The return receipt will be accessible via the certified tracking number from the USPS homepage. However, if you would like to receive the signature PDF via email, follow these steps:
  • Visit
  • Enter the Certified tracking number into the Track a Package box
  • Click the "Return Receipt Email" dropdown and fill the required fields in the form
  • The USPS will send you an email with the signature file like the below example

Registered Mail

Registered Mail is an add-on service offered by USPS that provides extra protection for high-value letters and packages. When you send a letter by Registered Mail, the USPS establishes a chain of custody that tracks and secures your shipment throughout the entire transit process—from the moment it is dropped off at the Post Office until the moment it’s delivered.
You can send letters as Registered Mail through the Lob API by passing the value registered in the extra_services parameter of the create letter request.
The USPS Registered Mail add-on is only available for First Class mail and can be sent and tracked both domestically and internationally.
Letters sent as registered will receive a different set of scan events compared to regular First or Standard Class mail. Registered Mail will instead receive a carrier tracking number and link, which is an add-on that will be available 3 business days following the mailer’s send_date. This tracking number can be used to track the mailer via the carrier’s website.