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.
Shared IPs create unpredictable deliverability. Multiple organizations send from the same IP address, meaning one sender’s poor practices damage reputation for everyone. For phishing simulations requiring consistent deliverability, shared IPs are inadequate.
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 focuses on establishing 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 increases 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. Any spike in bounces or complaints signals need to slow increase.
Day 8-14 reaches 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 scales 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 using tools like dnsbl.info and mxtoolbox.com/blacklists.aspx. Monitor spam complaint rates through feedback loops with major email providers. Watch sender scores via SenderScore.org and similar reputation tracking services.
If warming is interrupted for more than 30 days of inactivity, restart the process. Email providers view inactive IPs as potentially compromised or sold to spammers, requiring re-establishment of trust.
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. Phishing simulations should never share SMTP infrastructure with business-critical communications.
TLS encryption is mandatory for modern email delivery. Unencrypted SMTP connections are increasingly rejected by recipient servers or trigger spam filtering.
Authentication mechanisms (SMTP AUTH) verify sender identity and prevent unauthorized use of your SMTP server. Without authentication, your server becomes an open relay vulnerable to abuse by actual spammers.
Reverse DNS (PTR records) must match your sending server’s hostname. Mismatched reverse DNS is a common spam indicator checked by most filters.
Proper retry logic handles temporary delivery failures gracefully. Aggressive retry attempts trigger rate limiting and reputation damage. Implement exponential backoff for failed deliveries.
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, preventing simulation deliverability issues from affecting business email.
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. Test templates through spam filter checkers before campaigns.
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 rather than spam blasts.
Engagement-based targeting starts with employees most likely to engage positively. Early positive engagement (opening emails, clicking links as part of training) builds initial reputation before expanding to broader audiences.
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 from future campaigns.
Troubleshooting Common Deliverability Issues
Low open rates under 10% typically indicate spam folder placement. Check sender reputation using multiple reputation tracking services. Verify SPF/DKIM/DMARC configuration is correct. Review email content for spam triggers.
Blacklist placement blocks delivery entirely to affected recipients. Monitor blacklists daily using mxtoolbox.com or similar services. If listed, follow delisting procedures specific to each blacklist, which often require proving resolved issues causing listing.
Bounces exceeding 5% indicate list quality or technical issues. Hard bounces (invalid addresses) should be removed immediately. Soft bounces (temporary failures like full mailboxes) may resolve on retry but require monitoring.
Domain reputation issues affect all emails from your domain. Check domain-level reputation separately from IP reputation. Domain reputation problems often stem from DMARC policy issues or previous spam associated with your domain.
Microsoft 365 and Gmail have different filtering approaches. Gmail relies heavily on engagement metrics and machine learning. Microsoft 365 weighs authentication more heavily. Test campaigns with both providers before full deployment.
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 like Postfix or Poste.io, configuring authentication and TLS, implementing SPF/DKIM/DMARC, managing IP warming over 18+ days, monitoring reputation continuously, troubleshooting delivery issues, and maintaining infrastructure as email providers change requirements.
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 services alone cost $9-29/month. Add engineer time for setup (8+ hours), ongoing monitoring (2-4 hours monthly), and troubleshooting (variable but significant), and self-hosted SMTP quickly exceeds managed service costs.
For phishing simulations specifically, managed solutions designed for security testing understand the unique deliverability challenges and implement optimizations that general-purpose SMTP providers don’t offer.
Testing and Validating Email Deliverability
Before launching large campaigns, validate deliverability with small test sends to diverse recipients.
Seed list testing sends test emails to accounts across major providers (Gmail, Outlook, Yahoo) and checks inbox vs spam folder placement. Create test accounts specifically for this purpose, including both personal and business accounts.
Mail-tester.com provides automated analysis of email authentication, content, and technical configuration. Send test emails to the provided address and receive detailed scoring with specific improvement recommendations.
Inbox placement monitoring tracks where emails land across recipients. Tools like GlockApps or Email on Acid send test emails to hundreds of mailboxes and report exact placement rates.
Authentication verification tools like dmarcian.com validate SPF/DKIM/DMARC configuration and identify issues preventing proper authentication.
Reputation monitoring should be continuous, not one-time. Check sender reputation weekly using multiple services including SenderScore.org, TrustedSource.org, and Google Postmaster Tools.
Building Long-Term Deliverability Success
Sustainable high deliverability requires ongoing attention to sender reputation and email provider relationships.
Consistent sending patterns help maintain reputation. Irregular campaign schedules create suspicion. Establish predictable sending volumes and frequencies when possible.
Feedback loop participation with major email providers allows monitoring spam complaints directly. Gmail and Microsoft offer feedback loop programs providing real-time complaint notifications.
Suppression list management removes complainers and unengaged recipients from future campaigns. Continuing to send to people who marked emails as spam guarantees reputation damage.
Re-engagement campaigns for inactive recipients can improve engagement metrics or identify recipients to remove from lists. Send periodic campaigns asking inactive recipients to confirm they want to continue receiving emails.
Industry best practices evolve continuously. Email providers update filtering algorithms frequently. Subscribe to deliverability newsletters and participate in email deliverability communities to stay current.
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.
Stop fighting spam filters and start training employees. Get production-ready GoPhish with optimized email deliverability and launch campaigns that actually reach inboxes.
