SMTP Verification
Also known as: SMTP check, SMTP validation, mailbox verification
SMTP verification is the process of connecting to a mail server and issuing RCPT TO commands to check whether a specific mailbox exists and accepts mail — without actually sending a message. It catches addresses that pass syntax and DNS checks but would still bounce. SMTP results are combined with catch-all detection and reputation signals to produce a deliverability tier.
Related concepts
Email Verification
Email verification is the process of confirming that an email address exists and can receive mail before you send to it. A thorough pipeline runs syntax, DNS/MX, SMTP, catch-all, disposable, and role checks plus authentication analysis, then assigns a deliverability tier. Verifying first protects sender reputation and cuts bounce rates.
Email Deliverability
Email deliverability is the ability of your messages to reach the recipient's inbox rather than the spam folder or a hard bounce. It depends on authentication (SPF/DKIM/DMARC), sender reputation, list quality, content, and engagement. Verified data and a warmed-up sending domain are the foundation of strong deliverability.
Bounce Rate
Bounce rate is the percentage of sent emails that fail to deliver. Hard bounces are permanent failures (invalid address); soft bounces are temporary (full mailbox). High bounce rates signal poor list quality to mailbox providers and damage sender reputation, which is why verifying addresses before sending is essential.
Catch-All Domain
A catch-all (or accept-all) domain accepts mail to any address at the domain, even ones that don't exist, so an SMTP check can't conclusively verify a specific mailbox. Catch-all addresses carry elevated bounce risk and are typically scored as a separate, lower-confidence tier rather than treated as fully valid.
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