Lob Help Center

Mail send settings

As you use the Lob API to send mail programmatically, it is important that you manage your account settings to send mail so that they accurately reflect the needs of your business workflows and the frequency of your customer engagements and outreach. Most of the following settings can be adjusted via the API or in your Lob dashboard.

Mail use type

For any mail piece that is sent through Lob, you will need to indicate which mail use type is being sent. This helps Lob populate the right mail settings and postage options to ensure your mail is produced and delivered in an optimal way. 

Select either of the following use type for your mail pieces: 

  • Marketing mail: These are mail pieces sent for promotional purposes, such as for acquisition, cross-sell, retention, or win-back campaigns.
  • Operational mail: These are mail pieces sent for transactional purposes as part of your trusted business outreach and customer communications, such as account notifications, invoices and payment, policy changes, service announcements, or other similar reasons.  

US mail strictness settings

Depending on what business logic you’d like to take based on your verification result, you can toggle your US Address Strictness Settings in your dashboard. Each level of strictness determines how much you'd like for our platform to use internal data on deliverability to restrict mail from being created and delivered to potentially invalid addresses. There are 3 possible Strictness Levels:

  • Strict: Only US addresses that Lob and the USPS deem deliverable can be successfully used as the to address for postcard, letter or check resources. Mail piece requests sent with non-deliverable addresses will return a 422 (Unprocessable Entity) error. This maps to a deliverability value of deliverable (see API documentation). If addresses have never been run through verification (i.e. were created in the past), they will also err when used as the to address.
  • Normal: Mail pieces will be created for addresses that Lob and the USPS deem deliverable, as well as addresses for which secondary information is extraneous or missing. Otherwise, you will receive a 422 (Unprocessable Entity) error. This maps to the deliverability values of deliverable, deliverable_unnecessary_unit, deliverable_incorrect_unit and deliverable_missing_unit (see API documentation). If addresses have never been run through verification (i.e. were created in the past), they will also err when used as the to address.
  • Relaxed: All mail pieces will be successfully created and mail delivery will be attempted, regardless of address validity. You will not receive a 422 (Unprocessable Entity) for Address Strictness reasons.

Ultimately, this allows you completely control the destiny of your final pieces. Only want to send to addresses that are guaranteed to be deliverable? Pick "Strict" mode. Confident that your addresses are right and want us to mail them out anyway? Use "Relaxed" mode. We want to give you the power to decide when Lob should be sending mail or when we should be rejecting based on deliverability. Ultimately, this reduces the amount of undeliverable mail you send and saves you money.

Merge variable strictness setting

Depending on how much control you'd like over your HTML integration, we offer two different account settings that affect how we treat merge variables. This account setting affects the POST /v1/postcards, POST /v1/self_mailers, POST /v1/letters, and POST /v1/checks endpoints in both test and live mode:

  • Strict: Lob will send a 422 error if you define a merge variable in your HTML that is not passed in the merge_variables field of that request. Pass '' or null to have a particular defined variable not render.
  • Relaxed: Lob will not send an error if you define a merge variable in your HTML that is not included in the merge_variables field of that request. Instead, we will simply render nothing in the HTML in place of that merge variable.

Regardless of your strictness setting, if you pass merge variables keys that are not defined in your HTML, no error will be thrown. Your HTML will simply be rendered as normal without substituting the extra variable(s).

NOTE: On the 2020-02-11 and later API versions which support JSON in merge variables, merge variable strictness will still apply to the nested object keys, i.e. if a nested merge variable is undefined on the strict setting, then Lob will send an error.

Cancellation windows

By default, all new accounts have a 5-minute buffer or cancellation window for any single mail piece that is created. Within this timeframe, you can cancel mailings, free of charge. This gives you the flexibility to quickly QA your mailings before they are finally sent to production.

Note that in the Campaigns feature, you have additional flexibility to establish cancellation windows specific to each campaign, which will override your account-level cancellation window settings.

Once the cancellation time window has passed for any single mail piece or for a batch of mail pieces within a campaign, the mailing is no longer cancelable and has been sent to our printers for production. 

Note that customers on higher editions can customize their cancellation windows by mail product in their dashboard settings. Upgrade to the appropriate Print & Mail edition if interested in gaining access to this ability. 

Editing your windows

Growth edition customers and above can customize their cancellation windows. If you have access to this feature, your cancellation window can be anything from 0 minutes (no cancellation window) to up to 3 days.

Keep in mind that when you edit your cancellation window settings, any changes made will only apply to mailings created after the update was made. If you find yourself constantly changing your cancellation window for different use cases, we recommend using our scheduling mailings‍ feature instead.

Canceling your mailings

Within your chosen cancellation window, any single mail piece or batch of mail pieces within the Mail Campaign feature is cancelable. This means that they will be completely removed from production and that they will not count towards your monthly usage for billing purposes.

To cancel a mailing, either use the API endpoint or cancel the mailing from the dashboard. In either case, the mailing will only be cancelable if its send_date has not yet passed.

To cancel a list of mailings, you can use a combination of our LIST and DELETE endpoints to paginate through results and attempt to delete mailings that meet the specific criteria you set.

Bypassing windows with scheduled mailings

Even if you have a cancellation window set on your account, using the scheduling mailings‍ feature to schedule a mailing will override your cancellation window and the send_date passed will be used instead.

Not only is this useful for scheduling mailings far off in the future, but it is also handy for completely bypassing any cancellation window you might have and sending one mailing or batch off to production immediately.

Best practices 

  • Upgrade to a Growth edition or above to take advantage of configurable cancellation windows. Adjust this window to a time that works for you so you have time to cancel on your end
  • Have your developer build in buffer time into your Lob integration to give your team extra buffer time to cancel before submitting a live production request to Lob
    • Protip: configure different cancellation windows per mail format / use case individually in your dashboard settings
  • If you were unable to stop a check sent through Lob, you can still prevent it from being deposited by contacting your bank and placing a stop payment on the check. This can be done by contacting and providing your bank with the check number.
  • Give yourself a longer buffer while your team becomes familiar with the Lob platform, especially when sending through the Mail Campaigns feature, as sending large-scale mail volumes may amplify the magnitude (and costs) of any errors committed. Once you've become familiar with Lob and its functionalities, gradually adjust your buffer time in order to send your mail to production sooner. 
  • Cancellation windows on your account will delay the sending of any outgoing mail by the desired time window. Make sure to account for cancellation windows when considering daily production cutoff times of 10am PT.
    • For example, if you want to make the cutoff time of 10am PT and your cancellation window is 60 minutes, your cutoff time is actually 9am PT. 
    • If you want to bypass your cancellation window for a certain send, you can do so by scheduling your mailings.  

Idempotent requests

Idempotent Requests are requests that can be called many times without producing different outcomes. GET and DELETE requests are idempotent by definition, meaning the same backend work will occur no matter how many times the same request is issued. POST requests, however, are not idempotent. Sending a successful POST request once will result in a newly created object. If you send the same POST request 5 times, you will create 5 resources, assuming none of those requests err. If a network error occurs, there is no deterministic way to ensure the exact number of resources created.

For this reason, we have added a feature that will allow you to safely resend the same POST request to /v1/postcards, /v1/self_mailers, /v1/letters, or /v1/checks and ensure that duplicate products are not created. To perform an idempotent POST request, you simply need to provide an additional Idempotency-Key header that uniquely identifies that resource. See our API documentation for more specific information.

Generating idempotency keys

The Idempotency Key is a feature that allows you to pass a unique key along with each request and guarantee that only one mailer is created and sent to prevent any duplicate mailings from being created. You can safely retry the same request with the same Idempotency Key and be assured that no duplicates are created even if the API is called multiple times within 24 hours.

We suggest using V4 UUID or another appropriately random string, but how you create the unique keys is up to you. For example, if you would like to associate a check with an internal unique ID of the user you are sending the check to, you may use that user id and the transaction date as an Idempotency Key (as long as you can guarantee the uniqueness of each Idempotency Key across requests to the same product). This key will expire after 24 hours, meaning if you resend the same request with the same Idempotency Key after 24 hours, a second resource will be created. Ultimately, it is up to you to make sure that you are appropriately setting the uniqueness of your keys based on your business logic. See resources below for help generating V4 UUIDs in various languages.

Sending requests on retry

In case of failure, we recommend following something akin to an exponential backoff algorithm for retrying your idempotent requests. This ensures that you aren't retrying continuously on a downed server, thereby contributing to the issue at hand.

Scheduling mailings for a future date

Mailings can be scheduled up to 180 days in the future. Until that time, mailings can also be canceled. You can use this feature to:

  • Create automated drip campaigns (e.g. send a postcard at 15, 30, and 60 days)
  • Schedule recurring sends
  • Plan your mailing schedule ahead of time

Up until the time a mailing is scheduled for, it can also be canceled. If you use this feature in conjunction with a cancellation window, the send_date parameter will always take precedence.

NOTE: This feature is exclusive to certain customers. Upgrade to the appropriate Print & Mail edition to gain access.

Example Create request using send_date

  curl https://api.lob.com/v1/postcards \
    -u test_0dc8d51e0acffcb1880e0f19c79b2f5b0cc: \
    -d "description=Demo Future Postcard" \
    -d "to=adr_bae820679f3f536b" \
    -d "from=adr_210a8d4b0b76d77b" \
    -d "front=tmpl_b846a20859ae11a" \
    -d "back=tmpl_01b0d396a10c268" \
    -d "merge_variables[name]=Harry" \
    -d "send_date=2021-07-26"

Scheduling a future mailing

Scheduling a mailing for the future just requires one additional parameter: send_date. This specifies a date and time up to 180 days in the future to start processing the mailing. For billing purposes, requests will count towards the month of their send_date, and will not be charged if they are canceled before that time. If your account has a cancellation window set for the resource you are creating, passing a send_date will override that window. For more detailed request information, see our API documentation.

Canceling a future mailing

If something changes on your side and you no longer wish to send a mailing that had already been scheduled (such as the customer taking a certain action), you can programmatically cancel those requests as long as the mailing's send_date has not passed. Additionally, you can cancel a mailing from your dashboard on the page associated with your mail piece.


Below are resources we recommend in various languages for generating V4 UUIDs.

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