How Deliverability Software Checks SPF and DKIM

Introduction to Email Authentication Verification

In the modern ecosystem of digital communication, the journey of an email from sender to recipient is far more complex than a simple transfer of data. To combat the rising tide of phishing and spoofing, mail servers have implemented rigorous gatekeeping protocols. Two of the most critical are SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail). While these protocols are technical standards, most senders interact with them through deliverability software.

Deliverability software acts as a diagnostic bridge, simulating the checks that major inbox providers like Gmail or Outlook perform. By understanding how these tools verify your records, you can ensure your infrastructure is robust enough to bypass spam filters and land in the primary inbox. This guide dives deep into the algorithmic and technical processes deliverability software uses to validate SPF and DKIM.

The Anatomy of an SPF Check

SPF is a DNS-based mechanism that specifies which mail servers are authorized to send email on behalf of your domain. When deliverability software "checks" your SPF, it isn't just looking for the existence of a record; it is performing a multi-step validation of your domain's sending authority.

1. DNS Query and Record Retrieval

The software begins by issuing a DNS query for TXT records associated with your domain. It specifically looks for a string starting with v=spf1. If multiple records are found, the software immediately flags a critical error, as the SPF specification (RFC 7208) dictates that a domain must have exactly one SPF record.

2. Syntax Parsing and Recursive Lookups

Once the record is retrieved, the software parses the mechanisms (e.g., ip4, include, a, mx). The most complex part of this process is the recursive lookup. When a record contains an include:thirdparty.com statement, the software must go and fetch the SPF record for that domain as well.

Deliverability software is specifically designed to count these lookups. The SPF protocol has a hard limit of 10 DNS lookups. If your record requires 11 or more lookups to resolve all authorized IPs, the software will trigger a "PermError" (Permanent Error). High-end deliverability tools will provide a "lookup tree" to show exactly which service is pushing you over the limit.

3. IP Authorization Simulation

After flattening the record into a list of authorized IP addresses and ranges, the software simulates a delivery attempt. It compares the IP address of your actual sending server against the authorized list. If the IP is missing, the software predicts a "SoftFail" (~all) or "Fail" (-all) result, warning you that your emails are likely to be rejected or throttled.

How Software Validates DKIM Signatures

Unlike SPF, which is checked at the domain level, DKIM is checked at the message level. It uses asymmetric cryptography to ensure that an email hasn't been tampered with during transit. Deliverability software checks DKIM through two primary methods: record validation and signature verification.

1. The Selector Lookup

DKIM records live at a specific subdomain formatted as selector._domainkey.example.com. Since "selector" can be any string chosen by the sender, deliverability software must first identify which selector is being used.

When you send a test email to a deliverability tool, it extracts the DKIM-Signature header from the email. It looks for the s= tag (the selector) and the d= tag (the signing domain). Using these, the software performs a DNS query to find the corresponding public key.

2. Public Key Analysis

The software doesn't just check if the key exists; it analyzes its strength and format. Key points of inspection include:

Key Length: Modern deliverability software will flag 1024-bit keys as a security risk, recommending the more secure 2048-bit keys.
Syntax Errors: It checks for common mistakes like misplaced semicolons, extra spaces, or truncated keys that often occur during copy-pasting into DNS editors.

3. Cryptographic Hash Verification

This is the "heavy lifting" of DKIM verification. The deliverability software takes the incoming test email and performs the following steps:

Canonicalization: It prepares the email headers and body according to the rules specified in the signature (either relaxed or simple). This accounts for minor changes like whitespace adjustments that might happen during transit.
Hashing: It runs the specified algorithm (usually sha256) on the email content to generate a local hash.
Decryption: It uses the public key found in your DNS to decrypt the digital signature provided in the email header.
Comparison: If the locally generated hash matches the decrypted hash, the DKIM check passes. A mismatch indicates that the content was altered or the keys are misconfigured.

Common Errors Detected by Deliverability Software

When using professional-grade tools, you will often encounter specific error messages. Understanding these is key to maintaining a high sender reputation.

SPF "Too Many DNS Lookups"

This is the most common error for growing companies. As you add more tools (CRMs, Help Desks, Marketing Platforms), each include adds to your count. Deliverability software helps identify redundant includes or suggests SPF flattening, which converts domain-based records into static IP lists.

DKIM "Body Hash Did Not Verify"

This error suggests that the email content changed after it was signed but before it reached the recipient. This can happen if a mail transfer agent (MTA) adds a footer, modifies tracking links, or changes the encoding. Deliverability software helps pinpoint where in the "hop" sequence the modification occurred.

The "Alignment" Check

Both SPF and DKIM require DMARC alignment to be truly effective. Deliverability software checks if the domain in the From: header matches the domain used in the SPF record (Return-Path) and the DKIM signature. If they don't match, even "passing" records won't help you stay out of the spam folder.

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The Software Workflow: From Test to Report

To give you a clear picture of the technical flow, here is how a typical deliverability software suite processes your request:

Ingestion: The user sends a test email to a unique address provided by the software.
Header Extraction: The software parses the Authentication-Results and Received headers.
Real-time DNS Check: The software queries live DNS servers (not cached results) to see what a recipient's server sees at that exact moment.
Policy Evaluation: The software compares the results against the domain's DMARC policy (p=none, p=quarantine, or p=reject).
Reputation Cross-Referencing: Often, the software will also check if the authenticated IPs or domains are on any major blocklists (RBLs).

Feature	SPF Check	DKIM Check
Primary Goal	Authorize Sending Source	Verify Message Integrity
Location	Domain DNS	Email Header + DNS
Software Focus	Lookup limits & IP matches	Cryptographic match & Key length
Common Fix	Flattening or removing includes	Re-generating 2048-bit keys

Conclusion

Deliverability software is an essential ally in the fight for inbox placement. By meticulously simulating the SPF lookup process and the cryptographic verification of DKIM, these tools allow you to find and fix infrastructure flaws before they impact your campaign's ROI. Ensuring your records are not only present but syntactically correct and properly aligned is the foundation of a professional email strategy. Regular audits with deliverability software ensure that as your tech stack grows, your emails continue to reach their intended destination.