After reviewing 300+ campaigns and sending over 2 million cold emails, Gabriel Martínez, Founder of Reachflow, finally uncovered what actually keeps emails out of spam — even in 2026.
For agencies running outbound for clients, one truth is clear: deliverability is everything. You can have the best copy, the strongest offer, and perfect timing — but if your emails never reach the inbox, none of that matters.
Without a solid infrastructure and warm-up process, even the best offers and copy fail to convert.
This guide breaks down a step-by-step deliverability audit framework built for agencies operating at scale, designed to help you improve email deliverability. Plus, we’ve included a free checklist to identify and fix weak points in your current cold email setup.
Why Deliverability Is the Foundation of Cold Email Success
Think of a sender reputation as your email credit score.
If Gmail or Outlook doesn’t trust your domain, your messages won’t reach the inbox — no matter how great your campaign is.
Deliverability starts with trust. That trust is built gradually, through proper authentication, domain structure, and behavioral signals that mimic genuine human interactions.
A single misconfiguration, or too many unverified domains, can drop that score instantly.
When that happens, even perfect copy won’t save you.
In 2025, the game changed.
Smartlead data shows:
Step 1: Build Reputation Before Volume
The first rule of deliverability: never scale before warming up.
Every new domain should start small: send 5–10 emails per day, gradually increasing to 30–40 per inbox over 2–3 weeks. This slow ramp-up signals to mailbox providers that your messages are legitimate, not mass blasts.
Pro Tip: AI-powered warm-up tools, like Smartlead, simulate real conversations, generate opens, replies, and even move emails from spam to inbox automatically — strengthening sender reputation with minimal manual effort.
Gabriel mentioned “Even when your domains are performing well, you should never stop warming up the mailboxes. And smartly allows you to keep a low-level background warm-up. And smart lead allows you keep a low-level background warm-up running indefinitely. We always leave it on because it maintains a domain reputation even during campaign policies. And it's basically an insurance policy for your eBox placement. And this is how we consistently maintain between 97 and 95% inbox rate month after month. That's the complete smart warm-up and delivery framework we use every single day.”
Step 2: Perfect Your Email Infrastructure
Before any sending begins, infrastructure must be flawless. There are four non-negotiable pillars:
- Authentication: SPF, DKIM, DMARC, and MX records must be correctly set up and verified. Proper authentication signals legitimacy and significantly increases inbox placement.
- IP Strategy:
- Small senders: shared IPs are sufficient
- High-volume senders: dedicated IPs isolate reputation and reduce risk
- Small senders: shared IPs are sufficient
- Domain Setup: Always send from secondary domains, never your primary company domain. Reachflow uses 10–20 secondary domains, each with 2 inboxes to safely distribute volume.
- Sending Behavior: Keep daily limits under 30–40 emails per inbox, randomize send times slightly, and disable tracking during warm-up to avoid spam flags.
Step 3: Optimize Smartlead Configuration
Once the foundation is ready, here’s the proven Smartlead configuration Reachflow uses across campaigns:
- Start warm-up at 5–10 emails/day and ramp up to 30–40/day.
- Maintain a 30–40% reply rate during warm-up.
- Disable tracking for links and opens during both warm-up and early sending.
- Continue background warm-up indefinitely — it acts as deliverability insurance.
Smartlead’s SmartServers and SmartDelivery automatically pause underperforming inboxes, randomize send patterns, and isolate domain reputation, ensuring no shared contamination between users.
Step 4: Content Quality Still Matters
Even with perfect setup, poor content can trigger spam filters. During warm-up and early campaigns, focus on plain text emails, avoid heavy links, images, or formatting.
Each email should read like a real conversation:
- Personalized greetings
- Natural sentence structures
- Genuine signatures
Mailbox providers reward human-like communication. Automation that looks human performs best.
Smartlead’s analysis of 3.2 million cold emails found:
Emails under 120 words perform 38% better
Subject lines with 4–6 words yield 22% higher open rates
Plain text beats HTML by 42%
Step 5: Segment and Clean Your Lists
A deliverability audit isn’t complete without list hygiene.
Here’s what to review:
- Engagement segmentation: Separate active users from inactive ones.
- Validation: Use tools to verify all addresses and remove bounces weekly.
- Avoid role-based emails: Skip addresses like info@, sales@, admin@.
- Opt-in preference: Prioritize leads who have shown previous engagement (LinkedIn activity, website visits, or form fills).
Clean lists mean consistent deliverability.
Step 6: Monitor, Iterate, and Maintain
Deliverability isn’t a one-time setup — it’s an ongoing process. Smartlead’s deliverability dashboard lets you track inbox placement, engagement, and bounce rates across all campaigns.
If performance drops:
- Pause campaigns briefly.
- Recheck authentication records.
- Restart warm-up on affected inboxes.
- Reduce sending volume for a few days.
And never stop background warm-ups — they keep your domain reputation active, even when campaigns pause.
The Deliverability Audit Checklist (Free)
1. Infrastructure & Authentication
- SPF, DKIM, DMARC, MX setup verified
- Dedicated or clean shared IPs
- Secondary domains active
2. Sending & Warm-Up
- Warm-up volume between 5–40 emails/day
- No tracking during early sends
- Background warm-up always on
3. List & Engagement Management
- Bounce rate under 3%
- Engagement segmentation active
- Avoid role-based accounts (info@, admin@, sales@)
The Bottom Line
The best agencies aren’t just writing better copy, hey’re engineering trust into their outbound systems.
Once your deliverability is stable, everything else improves: higher open rates, consistent replies, and predictable pipeline growth. Your campaigns deserve to land where they belong, in the inbox.
Building a strong foundation takes time and attention, from proper infrastructure and warm-up to list hygiene and human-like personalization.
By following a structured deliverability audit and maintaining best practices, agencies can maximize campaign performance while protecting their reputation. Investing in these processes isn’t just about avoiding spam folders, it’s about creating a reliable, scalable outbound system that consistently drives results.
Frequently Asked Questions (FAQs)
1. How long does it take to build a sender reputation for a new domain?
Building a strong reputation typically takes 2–4 weeks of consistent warm-up, starting with 5–10 emails per day and gradually increasing to 30–40 per inbox. The exact timeline depends on volume, engagement, and infrastructure quality.
2. Can I send cold emails from my primary company domain?
It’s not recommended. Sending from your primary domain risks your main business communications. Always use secondary domains for cold outreach, with 2+ inboxes per domain to safely distribute volume.
3. Do I need a dedicated IP for small-scale campaigns to improve email deliverability?
Not necessarily. Shared IPs are sufficient for small campaigns with low volume, though they carry slightly higher risk since reputation is shared. Dedicated IPs are only essential for high-volume campaigns that need to improve email deliverability at scale.
4. Should I use images, links, or HTML formatting in cold emails?
Plain text emails perform best for deliverability, especially during warm-up. Keep emails conversational, short, and human-like. Images, HTML, or heavy links may work if tested carefully, but they increase the risk of spam filtering.
5. How often should I clean and segment my email lists to improve email deliverability?
Regular list hygiene is critical. Validate emails weekly, remove bounces and duplicates, segment active versus inactive leads, and avoid role-based accounts like info@ or admin@. Prioritize opt-in leads to consistently improve email deliverability and engagement.
6. What is background warm-up, and why is it important?
Background warm-up continuously sends small amounts of engagement-driven activity (opens, replies, moving emails from spam to inbox) to maintain domain reputation even when campaigns are paused. Think of it as insurance for inbox placement.
7. How do Smartlead’s SmartServers and SmartDelivery protect my campaigns and improve email deliverability?
Smart Servers isolate your IP and domain reputation, while Smart Delivery dynamically adjusts sending patterns, pauses underperforming inboxes, and randomizes email elements (greetings, sentence structure, punctuation). This ensures your emails appear human and protects your reputation from shared contamination.
8. How can I monitor deliverability effectively?
Use a dashboard to track inbox placement, bounce rates, and engagement. If performance drops, pause campaigns, check authentication records, restart warm-up on affected inboxes, and reduce sending volume temporarily.
Author’s Details
Rajashree
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