Email verification FAQ, answered without ambiguity.

MailCheckup performs a four-stage sequential verification process: (1) Syntax & Format Inspection adhering to RFC 5321 and RFC 5322; (2) DNS & MX Record Resolution confirming operational mail exchange servers via RFC 1035; (3) Disposable & Role Flagging comparing the domain against a maintained blocklist of 97,000+ burner domains; and (4) Direct SMTP Mailbox Interrogation connecting to the recipient mail server and executing a RCPT TO protocol handshake to determine whether the mailbox exists, without ever sending an email message.

No. An outcome of 'Deliverable' means the receiving mail server returned an affirmative 250 OK response during SMTP handshaking, confirming the recipient mailbox is active and accepting messages. Inbox placement (whether an email lands in the Primary tab or Spam folder) is determined downstream by spam filters, sender domain reputation, SPF/DKIM/DMARC authentication, message body content, and recipient engagement.

An 'Unknown' result indicates the remote mail exchange server did not provide a definitive yes-or-no verdict—typically due to greylisting deferrals (451/421 codes), anti-abuse rate limits, firewall dropouts, or temporary DNS timeouts. Under MailCheckup's $0 Free Unknown Guarantee, inconclusive outcomes cost 0 credits ($0). Any credit hold placed at the start of the job is immediately released back to your balance.

A catch-all domain is configured with wildcard routing that accepts incoming messages addressed to any local-part, regardless of whether a specific individual mailbox exists. MailCheckup flags these domains as Catch-All (Risky). Because the mail server accepts every test address, SMTP interrogation cannot prove individual validity. We recommend isolating catch-all contacts into secondary, low-volume outbound sequences.

MailCheckup identifies throwaway and temporary burner inboxes by comparing the domain against an in-memory radix trie of over 97,000+ disposable email domains. This blocklist is automatically synchronized every 6 hours from canonical threat feeds and the primary GitHub disposable domains repository. Flagged addresses are classified as Disposable (Risky), enabling you to exclude them from marketing campaigns and SaaS trial signups.

Role Accounts are generic departmental addresses (such as info@, support@, billing@, or sales@) managed by multiple individuals. While deliverable, they carry high spam complaint rates and lower outbound reply rates. Mailbox Full indicates the recipient mailbox exists but cannot accept incoming messages because its storage quota is currently exhausted.

Exactly one credit is deducted for each finished chargeable outcome: Deliverable (valid), Undeliverable (invalid/dead), Catch-All, Disposable, Role Account, or Mailbox Full. Inconclusive outcomes—such as Unknown, Greylisted, Mail Server Unreachable, Provider Blocked, or System Error—are 100% free ($0) with zero credit deductions.

No. Purchased credit packs never expire. Whether you buy 5,000 credits for $15 or a wholesale agency pack of 500,000 credits, your credits remain permanently in your workspace ledger until consumed. Only credits included with recurring monthly subscription plans expire at the end of the active billing period.

When a verification job begins, MailCheckup calculates the maximum potential credits required and places a temporary hold on your workspace balance. As rows finish processing, confirmed chargeable results consume 1 credit each. If an address returns an unknown, greylisted, or timed-out verdict, no credit is consumed and that portion of the hold is immediately released back to your available balance.

We maintain a voluntary 7-day refund request window following any credit purchase. If you experience technical defects or are unsatisfied with the verification service, submit an inquiry to billing@mailcheckup.net within seven days of payment. Unused credits from the eligible transaction will be refunded through Stripe Checkout.

MailCheckup automatically identifies duplicate email addresses within a single list upload. The unique address is verified once against remote mail exchangers and billed once (1 credit). Upon export, the verified verdict is mapped across all duplicate instances in your file, preventing unnecessary billing.

All payments are processed securely via Stripe Checkout supporting major credit cards. Invoices and receipts are issued by Secuno LLC (Sheridan, Wyoming, USA) using invoice identifier format MCU-YYYYMMDD-####. Invoices can be downloaded directly from the customer billing portal.

You can upload files in CSV (comma-separated), TXT (one email per line), or paste addresses directly into the portal text area. We recommend UTF-8 character encoding. Verified results can be exported as segmented CSV, TXT, or Excel-compatible XLS files.

Yes, completely. MailCheckup never modifies, strips, or reorders your original columns. Our processing pipeline appends verification columns (verification_status, reason_code, checked_at) to the right side of your original schema, ensuring 100% compatibility when re-importing into CRMs or sequencers.

Yes. MailCheckup supports dynamic queue appending. You can upload additional addresses or append rows to an in-progress verification job without having to cancel, pause, or spin up a separate secondary list.

Yes. Because initial unknown outcomes cost $0 free, you consumed no credits on those contacts. You can export the 'Unknown' segment and rerun it hours later after remote greylisting deferrals expire. If the second run yields a definitive Deliverable or Undeliverable answer, standard 1-credit billing applies for the completed verdict.

Yes. Our single-check REST verification API endpoint (POST /v1/verify/single) delivers verification verdicts in under 180ms (p95). It is engineered specifically for SaaS trial forms, mobile registrations, and ecommerce checkouts, and supports client-side timeout budgets with a recommended fail-open pattern.

MailCheckup provides cryptographic HMAC-SHA256 signed webhooks that push list.completed and batch email.valid notifications directly to your HTTPS endpoint. For AI workflows, our native Model Context Protocol (MCP) server (cp.mailcheckup.net/mcp) allows agents in Cursor, Claude Desktop, and ChatGPT to run list verifications natively.

Strictly NO. MailCheckup acts solely as a Data Processor under GDPR Article 28. Uploaded email lists belong exclusively to your workspace. We never sell, share, monetize, cross-reference, or use your uploaded customer contacts to build third-party marketing databases.

Verification results and uploaded files remain accessible in your workspace for download for the duration of your configured retention period (standard 30 days). Workspace owners and administrators can purge verification jobs and downloaded results immediately from the portal at any time.

Workspace API keys begin with mck_ and are displayed only once upon generation in the customer portal. Webhook secrets (starting with whsec_) are stored hashed at rest. Webhook requests include an x-mailcheckup-signature header containing the HMAC-SHA256 digest of the raw payload bytes.

Yes. MailCheckup supports multi-user workspaces with strict role-based access control (Owner, Admin, Member). API keys and webhooks operate with workspace-level isolation, ensuring team members cannot access data belonging to other organizations.

Monthly recurring subscriptions can be cancelled with one click inside portal Billing settings. Cancellation immediately stops future renewals while leaving existing credit pack balances active. Full account deletion and data scrubbing under GDPR Article 17 can be requested via workspace settings or by emailing support@mailcheckup.net.

MailCheckup is operated by Secuno LLC, registered at 30 N Gould St, Sheridan, Wyoming, USA 82801. Our technical support team is reachable at support@mailcheckup.net, billing inquiries are handled at billing@mailcheckup.net, and telephone support is available at +1 (315) 961-7074.