How Email Verification Works

Email verification is a multi-step process that determines whether an email address is valid, active, and capable of receiving messages. Mailthentic performs up to six layers of checks on every address.

Step 1: Syntax Validation

The email is checked against RFC standards to ensure it follows the correct format: a local part, an @ symbol, and a valid domain. Addresses with illegal characters, missing components, or excessive length are immediately flagged as invalid.

Step 2: Domain & DNS Lookup

Mailthentic queries DNS records for the email's domain to verify:

• The domain exists and resolves.
• MX (Mail Exchange) records are configured, indicating the domain can receive email.
• If no MX records exist, it falls back to checking A/AAAA records.

Step 3: MX Provider Detection

The MX records are analyzed to identify the email provider (Google Workspace, Microsoft 365, Yahoo, ProtonMail, etc.). This matters because some providers — particularly Google and Microsoft — always return a "250 OK" response during SMTP checks, even for nonexistent mailboxes.

Step 4: SMTP Mailbox Verification

Mailthentic connects to the domain's mail server on port 25 and simulates the beginning of an email delivery:

1. HELO — Introduces our server.
2. MAIL FROM — Specifies a null sender (standard for verification).
3. RCPT TO — Asks the server if the target mailbox exists.

No actual email is sent. The server's response code tells us whether the mailbox is valid:

• 250 — Recipient accepted (mailbox likely exists).
• 550/553 — Recipient rejected (mailbox does not exist).
• 450/451 — Temporary failure (greylisting or rate limiting).
• 421/452 — Server busy or rate limiting.

Step 5: Catch-All Detection

To determine if a domain accepts all email regardless of the recipient, Mailthentic sends probe requests to randomly generated, non-existent addresses. If the server accepts them, the domain is flagged as a catch-all, which reduces confidence in individual mailbox verification.

Step 6: DNS Security Assessment

Finally, Mailthentic checks for the presence and configuration of:

• SPF (Sender Policy Framework) — Authorizes which servers can send mail for the domain.
• DKIM (DomainKeys Identified Mail) — Cryptographic signature for message integrity.
• DMARC (Domain-based Message Authentication) — Policy for handling authentication failures.

Missing or misconfigured records are flagged as risk factors.