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How is SPF Verified?

Published in Email Authentication 3 mins read

SPF (Sender Policy Framework) verification is a crucial step in email authentication. It ensures that emails claiming to be sent from a particular domain actually originate from an IP address authorized by that domain. This process involves a series of checks performed by the receiving mail server.

The Verification Process

  1. DNS Lookup: The receiving server first performs a DNS lookup of the sender's domain, searching for a TXT record containing the SPF policy. This record lists the authorized IP addresses or mail servers permitted to send emails on behalf of the domain. (As stated in multiple sources, including EasyDMARC's SPF Record Checker and Lookup Tool, the process begins with a DNS lookup.)

  2. IP Address Check: Once the SPF record is retrieved, the server compares the sender's IP address, found in the "envelope from" address of the email header, against the IP addresses listed in the SPF record. (As noted by the provided reference material, the receiving mail server verifies if the "envelope from" address matches a valid IP address in the SPF record.)

  3. SPF Result: If the sender's IP address is listed in the SPF record, the SPF check passes. If the IP address is not found or there's a mismatch, the SPF check fails. A failed SPF check doesn't automatically mean the email is spam, but it's a strong indicator, and many mail servers will either reject the email or mark it as suspicious. (This aligns with the provided information showing that a mismatch results in an SPF test failure.) Tools like MXToolbox's SPF Record Check and Mimecast's SPF Record Check help diagnose issues with the SPF record itself.

Practical Examples

  • Successful Verification: An email from [email protected] sent from IP address 192.0.2.1 passes the SPF check if 192.0.2.1 is listed in the SPF record for example.com.

  • Failed Verification: An email from [email protected] sent from IP address 10.0.0.1 fails the SPF check if 10.0.0.1 is not listed in the SPF record for example.com.

This process helps reduce email spoofing and phishing attempts. While a failing SPF check isn't definitive proof of spam, it's a significant factor in email deliverability and recipient trust.

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