Why your emails land in spam (and what you can do about it). A complete guide to understanding the mechanics of email deliverability for MSPs who want to do outreach right.
Emails processed by Gmail daily — they know spam when they see it
Source: Google
of legitimate emails never reach the inbox
Source: Return Path
What is Deliverability?
Deliverability is the ability of your emails to reach the inbox — not spam, not bounced, not blocked.
The journey of an email:
- 1. You click send
- 2. Your email server sends to recipient server
- 3. Recipient server checks: Should I accept this?
- 4. If accepted: Inbox or spam folder?
- 5. Recipient sees (or does not see) your email
At each step, things can go wrong.
The Three Gatekeepers
1. Email Service Providers (ESPs)
- • Gmail, Microsoft 365, Yahoo
- • They decide inbox vs spam
- • They maintain blocklists
- • They track reputation
2. Spam Filters
- • Analyze content, sender, patterns
- • Score emails for spamminess
- • Block or filter based on score
3. Recipients
- • Mark as spam → Trains the system
- • Ignore/delete → Low engagement signals
- • Engage → Positive signals
What Affects Deliverability
Domain Reputation (Most Important)
- • Built over time through sending behavior
- • Damaged by bounces, complaints, spam traps
- • Affects ALL email from that domain
- • Can take months to rebuild
IP Reputation
- • The server/IP address sending your email
- • Shared IPs = shared reputation (risky)
- • Dedicated IPs = your reputation only
- • Matters more for high volume
Email Authentication
- • SPF: Proves you are allowed to send from this domain
- • DKIM: Proves email was not altered in transit
- • DMARC: Tells servers what to do with failures
- • Without these = instant suspicion
Sending Patterns
- • Sudden volume spikes = suspicious
- • Inconsistent sending = suspicious
- • Gradual, consistent growth = trusted
Engagement Metrics
- • Open rates, reply rates, click rates
- • Time spent reading
- • Low engagement = spam signals
Content
- • Spammy words and phrases
- • Too many links or URL shorteners
- • Image-heavy emails
List Quality
- • Bounce rate (invalid addresses)
- • Spam trap hits
- • Purchased lists = disaster
The Metrics That Matter
| Metric | Good | Dangerous | You are Cooked |
|---|---|---|---|
| Bounce Rate | <2% | 2-5% | >5% |
| Spam Complaints | <0.1% | 0.1-0.3% | >0.3% |
| Open Rate | >20% | 10-20% | <10% |
| Spam Trap Hits | 0 | Any | Multiple |
How Reputation Gets Damaged: The Death Spiral
- 1Send to bad list (bounces + complaints)
- 2Reputation drops
- 3More emails go to spam
- 4Engagement drops (people do not see them)
- 5Low engagement = more spam filtering
- 6Reputation drops further
- 7Eventually: blocked entirely
Common Causes:
- • Purchased or scraped email lists
- • Old, unverified data
- • No list cleaning before sending
- • Sending too much too fast
- • Ignoring unsubscribes
- • Spammy content
- • No authentication set up
Cold Email is Inherently Risky
Even with perfect execution:
- • You are emailing people who did not opt in
- • Some will mark spam (it is human nature)
- • Data is never 100% accurate (some bounces)
- • Engagement is lower than warm email
This is why cold email needs separate infrastructure.
Not because cold email is bad. Because the math is different.
The Authentication Trifecta
SPF (Sender Policy Framework)
- • DNS record that says these servers can send email for my domain
- • Without it: Anyone could spoof your domain
- • Receiving servers check this first
DKIM (DomainKeys Identified Mail)
- • Cryptographic signature proving email is legit
- • Proves email was not modified in transit
- • Adds trust signal
DMARC (Domain-based Message Authentication)
- • Policy telling servers what to do if SPF/DKIM fail
- • Options: none (monitor), quarantine, reject
- • Gives you visibility into spoofing attempts
All three are required for serious email sending.
Warming: The Patience Tax
New domains/mailboxes have NO reputation. No reputation does not equal good reputation. No reputation = unknown = suspicious.
Warming = gradually building sending reputation
The process:
- • Start with tiny volume (5-10 emails/day)
- • Send to engaged recipients (warmup services)
- • Gradually increase over 2-4 weeks
- • Build positive engagement history
- • Only then: Start real outreach (slowly)
Skip warming = Land in spam immediately
The Monitoring Imperative
You cannot fix what you cannot see.
Monitor:
- • Bounce rates per campaign
- • Spam complaint rates
- • Open rates (proxy for inbox placement)
- • Domain reputation (Google Postmaster Tools)
- • Blocklist status
React quickly:
- • Spike in bounces → Stop, clean list
- • Spam complaints rising → Stop, review content/targeting
- • Open rates tanking → Inbox placement issues
Do This
- Understand deliverability is a system with multiple gatekeepers
- Prioritize domain reputation above all else
- Configure SPF, DKIM, and DMARC on every sending domain
- Warm up new domains and mailboxes before sending
Avoid This
- Assume good intentions protect you from spam filters
- Send cold email from your production domain
- Skip warmup because you're impatient
- Ignore warning signs like dropping open rates
Pro Tip