Build around clear outcomes
Do not treat every failed request as an invalid email address. Keep API errors, temporary provider conditions, and verification outcomes separate in your application.
Start small, then automate
Verify a controlled test case first. Add timeouts and error handling. Then connect verification to the point in your workflow where it provides useful information without creating unnecessary friction.
Handle longer work deliberately
For supported asynchronous workflows, store the job identifier, check progress, and process results once they are available. Use documented retry and idempotency behavior to avoid accidental duplicate work.
Protect your keys
Call the API from a trusted server environment. Keep credentials out of browser code, public repositories, screenshots, and support messages. Rotate a key if you believe it has been exposed.
Make uncertainty part of your logic
An unknown result can mean the receiving provider did not give a conclusive answer. Your application may allow the user to continue, request ownership confirmation separately, or schedule a later review. Choose the behavior deliberately.
Is this an ownership check?
No. Confirm ownership through a separate challenge such as a verification link.
Is a browser API key safe?
Secret API keys should remain on your server.
Make email quality part of your workflow.
Validate at entry
Catch obvious input problems before they become support or delivery problems.
Bound the wait
Keep timeouts and provider uncertainty from trapping users in a signup form.
Process lists as jobs
Track longer operations separately from interactive requests.
Choose the right integration point
For signup forms, keep the user’s original input available and explain correctable mistakes. For imports, use a background job so a large file does not depend on a browser staying open. For CRM maintenance, associate every result with a stable contact record.
Separate a service error from an address verdict
An authentication error, quota failure, timeout, or unavailable service says nothing definitive about a mailbox. Keep transport errors outside your verification outcome field. Unknown outcomes should follow a deliberate application policy, not a default invalid branch.
Build for repeat delivery
Retries and duplicate events are normal integration conditions. Persist a request or job reference and make downstream updates safe to repeat. The production API contract must define authentication, idempotency, webhook signatures, rate limits, and billable outcomes before an integration is enabled.
Questions worth asking
Where do I run a list?
In the app at cp.mailcheckup.net. This site explains the product. The sample report does not check your file.
Does deliverable mean it will hit the inbox?
No. The mailbox server accepted the recipient when asked. It does not predict spam filtering or that a person will read the mail.