← Back to Blog

Introduction

You've configured GoPhish, created convincing phishing templates, and launched your first security awareness campaign. Then you check the dashboard: 5% email open rate. Your carefully designed simulation is failing because emails never reached employee inboxes.

Email deliverability is the invisible challenge that determines whether phishing simulations succeed or waste time. Even perfectly configured GoPhish campaigns accomplish nothing if Gmail, Outlook, and corporate spam filters block your emails.

This guide explains why security testing emails face higher delivery challenges than regular emails, provides detailed SMTP configuration for maximum deliverability, and shows you how to achieve 90%+ inbox placement for phishing simulations.

Why Security Testing Emails Face Unique Deliverability Challenges

Phishing simulations inherently trigger spam filters by design. They're testing whether employees can identify suspicious emails, which means incorporating elements that legitimate spam filters flag as potentially malicious.

Suspicious sender patterns are necessary for realistic testing. Simulations might spoof executive email addresses, use external domains similar to internal ones, or send from addresses employees don't recognize. Each technique triggers spam filter heuristics.

Unusual sending patterns complicate deliverability. Most email senders establish consistent patterns over time. Phishing simulations involve sudden bursts of emails to many recipients simultaneously, exactly the pattern associated with spam campaigns.

Link-heavy content with tracking pixels matches spam signatures. GoPhish templates include tracking URLs and invisible pixels to monitor opens and clicks. Spam filters scrutinize emails with high link-to-text ratios and tracking mechanisms.

Low engagement rates harm sender reputation. Legitimate marketing emails expect 20-40% open rates. Phishing simulations succeed when open rates are lower because employees are correctly identifying and avoiding suspicious emails. However, email providers interpret low engagement as indication that recipients don't want these emails.

The Foundation: IP Reputation and Sender Authentication

Email deliverability begins with IP reputation – the trust score email providers assign to sending servers based on historical behavior.

New IP addresses start with zero reputation. Email providers treat them cautiously because spammers constantly rotate to fresh IPs. Sending large volumes from new IPs triggers immediate spam filtering.

Dedicated IP addresses provide complete control over reputation but require proper warming and maintenance. Organizations own their sender reputation exclusively, preventing contamination from other senders.

IP warming is the 18-day process of gradually establishing positive reputation. Starting with small volumes and progressively increasing allows email providers to observe sending patterns and build trust.

SPF (Sender Policy Framework) records authorize specific IP addresses to send email for your domain. Without SPF, recipient servers can't verify that emails legitimately originate from your domain, often resulting in rejection or spam folder placement.

DKIM (DomainKeys Identified Mail) adds cryptographic signatures proving emails weren't tampered with in transit and verifying sender identity. Major email providers including Gmail increasingly require DKIM for inbox placement.

DMARC (Domain-based Message Authentication, Reporting & Conformance) policies tell recipient servers how to handle emails failing SPF or DKIM checks. Proper DMARC configuration improves deliverability while protecting your domain from actual phishing attempts using your brand.

The 18-Day IP Warming Process

IP warming can't be rushed. Aggressive sending from new IPs triggers permanent reputation damage requiring weeks to recover.

• Day 1-3: Establish baseline trust with minimal volume. Send 50 emails on day 1, 100 on day 2, and 500 on day 3. Monitor bounce rates (should be under 3-5%) and spam complaints (should be under 0.08%).
• Day 4-7: Progressively increase volume while monitoring metrics. Send 1,000 emails on day 4, 5,000 on day 5, 10,000 on day 6, and 20,000 on day 7.
• Day 8-14: Reach significant volumes suitable for most organizations. Continue increasing: 40,000, 70,000, 100,000, 150,000, 250,000, 400,000, 600,000 emails daily.
• Day 15-18: Scale to maximum required capacity. Send 1,000,000 on day 15, 2,000,000 on day 16, 4,000,000 on day 17, then double daily until reaching desired volume.

Monitoring throughout warming is critical. Track bounce rates, monitor spam complaint rates through feedback loops, and watch sender scores via SenderScore.org and similar reputation tracking services.

SMTP Configuration for Maximum Deliverability

Proper SMTP infrastructure separates successful phishing simulations from wasted efforts.

• Dedicated SMTP servers exclusively for security testing prevent cross-contamination with production email infrastructure
• TLS encryption is mandatory for modern email delivery
• Authentication mechanisms (SMTP AUTH) verify sender identity and prevent unauthorized use
• Reverse DNS (PTR records) must match your sending server's hostname
• Proper retry logic handles temporary delivery failures gracefully with exponential backoff

Advanced Deliverability Techniques

Subdomain segmentation isolates phishing simulation reputation from primary domain reputation. Configure simulations to send from security.example.com instead of example.com.

Content optimization balances realism with deliverability requirements. Include sufficient legitimate text content to avoid spam triggers. Avoid excessive capitalization, exclamation points, and spam-associated phrases.

Sending schedule optimization spreads campaigns across multiple days rather than burst sending. Distribute 1,000-recipient campaigns across 3-4 days to mimic natural email patterns.

List hygiene maintains clean recipient lists free from invalid addresses. High bounce rates permanently damage reputation. Validate email addresses before campaigns and immediately remove bouncing addresses.

Managed SMTP Solutions vs Self-Hosted

Building and maintaining SMTP infrastructure for phishing simulations requires expertise most security teams lack.

Self-hosted SMTP involves deploying mail servers, configuring authentication, implementing SPF/DKIM/DMARC, managing 18+ days of IP warming, monitoring reputation continuously, and troubleshooting delivery issues.

Managed SMTP services provide pre-warmed IPs with established reputation, configured authentication and compliance, deliverability monitoring and optimization, expert support for delivery issues, and automatic adaptation to email provider changes.

Cost comparison reveals managed services often cost less than self-hosted when accounting for engineering time. IP warming alone requires 8+ hours for setup and 2-4 hours monthly for ongoing monitoring.

Conclusion: Deliverability Determines Success

The most sophisticated phishing simulation fails if employees never see it. Email deliverability isn't a technical detail – it's the difference between effective security awareness training and wasted effort.

Organizations face a choice: invest weeks in SMTP infrastructure and IP warming, or leverage managed solutions with pre-established deliverability. For most security teams, infrastructure management distracts from core mission – improving security posture through effective training.

Managed GoPhish deployments include production-ready SMTP infrastructure with established sender reputation, achieving 90%+ inbox placement from day one. No IP warming required. No reputation monitoring. No deliverability troubleshooting. Just effective phishing simulations that reach employee inboxes.