Explore Deliverability & Verification Terminology
Search protocols, DNS records, and result codes, or select any term card below to launch the technical deep-dive inspector with RFC references and live code snippets.
SMTP (Simple Mail Transfer Protocol)
The standard communication protocol used by mail servers across the internet to transmit and route electronic mail messages.
Why Cold Emailers & Senders Care
Verification tools simulate an SMTP connection (HELO, MAIL FROM, RCPT TO) to test mailbox existence without actually sending an email or alerting the recipient.
Verification Capability Boundary
SMTP confirms whether the server accepts the recipient at that moment; it cannot predict whether future messages will land in the primary inbox or spam folder.
HELO mailcheckup.net
MAIL FROM:<probe@mailcheckup.net>
RCPT TO:<target@company.com>
250 2.1.5 Recipient OK
RSET
QUITSMTP (Simple Mail Transfer Protocol)
The standard communication protocol used by mail servers across the internet to transmit and route electronic mail messages.
MX Record (Mail Exchanger)
A DNS resource record that specifies the designated mail server responsible for accepting incoming email messages on behalf of a domain name, along with its priority preference.
SPF (Sender Policy Framework)
An email authentication standard that allows domain owners to publish a list of IP addresses and subnets authorized to send email on their domain's behalf using DNS TXT records.
DKIM (DomainKeys Identified Mail)
An email authentication technique that attaches a cryptographic digital signature to the message header, enabling the receiving server to verify that the email originated from the domain and was not altered in transit.
DMARC (Domain-based Message Authentication)
A policy protocol built on top of SPF and DKIM that tells receiving mail servers how to treat messages that fail authentication checks (none, quarantine, or reject) and specifies reporting addresses.
PTR Record & Reverse DNS (rDNS)
A DNS record that maps an IP address back to its associated domain name (the exact inverse of an A record). Forward-confirmed reverse DNS (FCrDNS) ensures both match.
STARTTLS (Opportunistic TLS)
An SMTP command extension that upgrades an existing plaintext communication channel into an encrypted TLS connection without changing ports.
RFC 5322 (Internet Message Format)
The formal Internet Engineering Task Force (IETF) specification defining the exact syntax, character encoding, and structure of email addresses and headers.
DNSBL / RBL (DNS Blackhole List)
A real-time database queried via DNS containing IP addresses and domain names known to originate unsolicited commercial email (spam) or malware.
Deliverable (Valid / 250 OK)
A verification outcome indicating that the recipient server verified the local mailbox exists and returned an explicit 250 OK status during the SMTP handshake.
Undeliverable (Hard Bounce / 550)
A permanent failure outcome where the recipient mail server explicitly rejects the mailbox as non-existent (e.g., 550 5.1.1 User unknown) or the domain lacks DNS records.
Catch-All (Accept-All Domain)
A mail server configuration where the domain accepts incoming email sent to any local mailbox prefix unconditionally, preventing outside servers from discovering whether specific mailboxes exist.
Disposable / Burner Email
A temporary, throwaway mailbox provided by services like Mailinator, Guerrilla Mail, or TempMail, designed to receive a single verification message before expiring.
Unknown Result ($0 Free)
An inconclusive verification outcome where the recipient mail server deferred the connection, timed out, or blocked probes with anti-spam firewalls, preventing definitive classification.
Greylisting (451 / 421 Deferral)
An anti-spam defense mechanism where a receiving MTA temporarily rejects connections from unfamiliar senders with a 4xx temporary error, expecting legitimate MTAs to retry later.
Mailbox Full (552 Storage Limit)
An outcome where the mailbox exists but cannot accept incoming mail because the user's storage quota has been completely exhausted (returning SMTP 552 or 452).
Role-Based Account
An email address tied to a corporate function, team, or department rather than a single individual (e.g., info@, support@, sales@, billing@, admin@).
Spam Trap (Honeypot)
A decoy email address operated by blocklist operators and ISPs (e.g., Spamhaus) designed to catch unsolicited senders and scraper bots. Traps never opt-in to email.
Sender Reputation & IP Warming
The algorithmic score assigned to a sending IP and domain by mailbox providers based on bounce rates, spam complaints, engagement metrics, and authentication records.
Inbox Placement Rate
The percentage of successfully accepted emails that land in the primary inbox of the recipient, as opposed to the spam/junk folder or promotions tab.
Hard Bounce vs Soft Bounce
Hard bounces (5xx) represent permanent failures (non-existent address/domain); soft bounces (4xx) represent temporary delivery failures (mailbox full, server down, rate limit).
Google & Yahoo 2024+ Mandates
Mandatory technical requirements enforced by Google and Yahoo since February 2024 for senders transmitting over 5,000 emails/day: mandatory SPF/DKIM/DMARC, one-click unsubscribe, and spam complaint rate below 0.3%.
Spam Complaint Threshold (<0.3%)
The ratio of recipients who flag an email as spam relative to total delivered messages. Google enforces a strict ceiling of 0.3% (with 0.1% recommended target).
Domain Throttling & Rate Limiting
When receiving mail servers artificially delay or defer message acceptance (using 421 Too many connections or 451 rate limits) due to excessive volume spikes or poor list quality.
Email Warmup Tool / Pool Risks
Automated peer-to-peer networks where secondary accounts send canned emails to one another to artificially boost reputation metrics.
Secondary Domain Isolation
The practice of hosting outbound sales cadences on dedicated variations of your primary corporate domain (e.g., getcompany.com instead of company.com) to insulate corporate email.
CSV Schema Preservation
The principle of verifying uploaded contact spreadsheets while keeping all custom CRM columns, lead IDs, tags, and UTF-8 formatting strictly intact in the export file.
Suppression List
A centralized negative database of email addresses that must never receive outreach due to past unsubscribe requests, spam complaints, or permanent bounce history.
Fail-Open Verification Pattern
An architectural pattern where a signup or checkout form allows user registration to proceed if the third-party verification API times out or returns an unknown status.
Webhook Event Notification (HMAC-SHA256)
An automated HTTP POST callback mechanism that notifies client servers when bulk verification lists complete, cryptographically signed with HMAC-SHA256 for integrity.
MCP (Model Context Protocol)
An open standard enabling AI assistants (like Claude, Cursor, and ChatGPT) to securely connect to external tools and verification APIs over streamable JSON-RPC 2.0.
Radix Trie / Blocklist Lookup
A space-optimized compressed prefix tree used to perform sub-millisecond lookups across tens of thousands of disposable and blacklisted domain names in memory.
Single Opt-In vs Double Opt-In
Single opt-in adds subscribers immediately upon form submission; double opt-in requires the subscriber to click a confirmation link sent to their inbox before activation.
GDPR Article 28 Compliance
The European data privacy framework establishing that MailCheckup acts strictly as a Data Processor on behalf of the customer (the Data Controller), with zero contact resale.
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