Understanding Validation Results

When NullBounce verifies an email, our engine performs a multi-layer diagnostic check. For your convenience, all technical parameters are combined into one primary verdict called Result, alongside granular attributes for advanced filtering.

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1. Primary “Result” Verdict

This is the primary status code designed to help you immediately decide whether it is safe to send to a recipient:

• Safe
  The recipient mailbox exists, MX records are correct, and the server accepted the simulated delivery handshake. Safe for primary sending domains.

• Invalid
  The address does not exist, the domain lacks mail servers, the mailbox is permanently closed or is disposable. Do not send to these addresses.

• Risky
  The email address is either Catch-All (accept-all domain) or a shared Deliverable Role-Based account (e.g. info@, sales@). They carry elevated risks of bounces or spam complaints.

• Unknown
  The destination mail server was temporarily unreachable, timed out, or anti-spam systems blocked the probe.

  • Dedicated IPs / Own MTAs: Usually acceptable to retry or send with caution.
  • Third-Party ESPs (Mailchimp, HubSpot, Sendgrid): Not recommended if strict bounce thresholds (< 2%) apply.
  • Billing: Unknown results are 100% free and consume zero credits.

• Aborted
  Appears only if you manually canceled a batch verification while it was processing. All unverified contacts are refunded back to your credit balance.

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2. Granular Diagnostic Attributes

In CSV exports and API responses, each email includes additional diagnostic attributes:

Parameter	Possible Values	Description
Deliverable	Valid Invalid Catch-All Unknown	Direct MTA/SMTP handshake deliverability result. Indicates whether the remote mail server confirmed mailbox existence.
Disposable	true / false	Indicates whether the email domain belongs to a temporary or burner provider (e.g., Mailinator, 10MinuteMail, GuerrillaMail). Don’t send – these emails are fake or temporary emails and are not safe for sending.
Role	true / false	Flags generic departmental inboxes (e.g., support@, billing@, jobs@). Associated with higher spam complaint rates.
Free	true / false	Flags consumer webmail providers (Gmail, Yahoo, Outlook.com, etc.). Helpful for B2B segmentation.

Deliverability Values Explained:

• Valid: The destination mail transfer agent (MTA) explicitly confirmed that the recipient mailbox exists and is ready to accept emails.
• Invalid: The address does not exist, the domain is not configured to receive email, or the mailbox quota is exhausted.
• Catch-All: The domain accepts incoming emails to any random username prefix. After an email is sent, the recipient’s mail server may silently discard the message or send a bounce message. For such domains, we cannot guarantee an email will be delivered to a user’s inbox. Sending to a Catch-All (also known as Accept-All) domain may increase your bounce rates and/or lower your open rates. Therefore, use a degree of caution when sending to Catch-All domains.
• Unknown: Destination server was slow or temporarily unavailable. Retrying after 5–10 minutes often yields a definitive result.

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3. Technical Sub-Statuses

When the Deliverable parameter returns Invalid or Unknown, NullBounce provides a granular reason in the substatus_text field (displayed in brackets in CSV reports, e.g. Invalid (No Smtp)):

Sub-statuses for Invalid Emails

Sub-status Reason	Technical Meaning
Failed Syntax Check	The email address violates standard IETF RFC formatting rules (missing @, invalid characters, incorrect dots).
Mailbox Not Found	The mail server responded with an error indicating the user mailbox does not exist (SMTP 550 User unknown).
Mailbox Quota Exceeded	The mailbox exists, but its storage is full. The server currently rejects incoming messages.
No Smtp	The domain has no valid MX records or lacks an active mail server.

Sub-statuses for Unknown Emails

Sub-status Reason	Technical Meaning
Failed Smtp Connection	The remote mail server refused or dropped the SMTP connection handshake.
Forcible Disconnect	The remote mail server disconnected immediately upon connection initialization.
Greylisted	The mail server uses temporary anti-spam greylisting. Re-validating after 5–15 minutes usually resolves this.
Mail Server Did Not Respond	The server accepted the initial connection but stopped responding to SMTP commands (HELO / RCPT TO).
Mail Server Temporary Error	The destination server returned a 4xx transient failure code (e.g., internal server load).
Timeout Exceeded	The mail server took too long to complete the verification handshake.
Antispam System	The destination domain operates aggressive perimeter filtering preventing third-party MTA probes.
Exception	An unexpected socket or network-level exception occurred during validation.