Most teams trying to improve email deliverability start with the wrong layer. They rewrite subject lines, swap out merge tags, change the call to action. All content-level fixes for an infrastructure problem.
Deliverability is an infrastructure problem before it is a content problem. Receiving servers decide inbox placement based on signals they verify before the email is even opened: where it came from, what domain reputation the sender has built, whether the message is authenticated. Content matters, but it matters last.
Here is the actual order of operations to fix and then improve email deliverability, top to bottom of the stack.
What is the first step to improve email deliverability?
Authenticate every sending domain. Without SPF, DKIM, and DMARC configured correctly, modern receiving servers (Gmail, Microsoft 365, Yahoo) treat the email as suspicious by default, and no content fix will compensate for that signal.
- SPF tells receiving servers which IPs and services are allowed to send on behalf of your domain. Most SPF problems come from too many "include" statements (the 10-lookup limit) or unflattened records that drift over time.
- DKIM signs each outgoing email with a cryptographic signature that proves the email was not modified in transit. If your DKIM signature fails, mail servers know the message is either forged or relayed by something untrusted.
- DMARC tells receiving servers what to do when SPF or DKIM fails (none, quarantine, reject) and where to send aggregate reports. A DMARC policy of "none" still produces reports that show exactly who is sending mail claiming your domain.
The practical check: send a test email from each sending domain to check-auth@verifier.port25.com or use Mail-Tester. If SPF, DKIM, or DMARC fails on any sending domain, deliverability is broken at the foundation. Authentication setup is week one work, not week six.
Why does domain and IP reputation affect inbox placement?
Receiving servers assign a trust score to your domain and your sending IPs independently, and they check that score before anything else. Bad reputation means spam folder by default, regardless of how clean your authentication is.
Tools to check:
- Google Postmaster Tools shows domain reputation, IP reputation, spam rate, feedback loop data, and delivery errors for any domain sending more than around 100/day to Gmail
- Microsoft SNDS gives IP-level reputation data for any IP sending to Outlook and Hotmail accounts
- MXToolbox / MultiRBL checks against 60+ public blocklists; if your IP or domain is on Spamhaus, SORBS, or Barracuda, that is the entire problem
If reputation is bad, the playbook depends on how bad. A single blocklist with a clear delisting process: submit the request, fix what flagged you, move on. Multiple blocklists or a sustained low Postmaster reputation: the domain is salvageable but it will take weeks of clean sending. A domain showing red across multiple sources for months: retire it. Checking domain reputation should happen before any other diagnostic.
How does email infrastructure affect deliverability?
A sending setup that works at 100 emails/day will fail at 1,000/day, and a setup that works for transactional email will fail for cold outreach. Most deliverability problems trace back to mismatched infrastructure, not bad copy.
The basic rules:
- Cold outreach: multiple secondary domains (never the main business domain), real provider inboxes (Google Workspace or Microsoft 365), warmed up properly before any campaign. SMTP relays are the wrong tool here regardless of price.
- Transactional email: dedicated transactional ESP (Postmark, Mailgun, AWS SES) on a subdomain like
mail.company.com, separate from everything else - Marketing and newsletter: dedicated marketing ESP (Mailchimp, Klaviyo, Customer.io) on a separate subdomain like
news.company.com - Internal and warm outreach: Google Workspace or Microsoft 365 on the main domain
Mixing these on a single domain is the most common cause of "deliverability got worse and we don't know why." A cold campaign that generates spam complaints damages the reputation of every other email type sharing that domain. See the full cold email infrastructure guide for how to set this up correctly.
How long does email warmup take to improve deliverability?
A properly run warmup takes 3-4 weeks for a new inbox or domain, and skipping it or rushing it is the single most common reason cold email campaigns fail on day one.
A new inbox or a new sending domain has zero history with receiving servers. No history means no trust, which means new senders default to spam folders or get rate-limited until trust is built.
Proper email warmup is the deliberate process of building that history: gradually increasing send volume over 2-4 weeks while generating positive engagement signals (opens, replies, marking as not-spam, marking as important). Warmup tools automate this with networks of inboxes that exchange mail with each other.
What kills warmup faster than anything: starting real campaigns mid-warmup. The volume jump from 30 warmup emails/day to 200 cold campaign emails/day is the exact pattern receiving servers flag as spam behavior. Finish warmup before any real send.
How does list hygiene improve email deliverability?
Bad lists are the most common cause of deliverability degradation on previously healthy domains. Three patterns matter, and letting any one of them run unchecked will tank inbox placement within weeks.
- Bounce rates above 2-3% tell receiving servers your list is dirty. This includes both soft and hard bounces, but hard bounces are the worse signal because they indicate addresses that never existed.
- Spam complaint rates above 0.1% are the threshold Gmail uses to start filtering more aggressively. One complaint per 1,000 sends. Keeping complaints below this requires verified opt-in for marketing and tight ICP targeting for cold outreach.
- Unverified addresses: every list, every month, run through an email verification service (NeverBounce, ZeroBounce, Bouncer). Verification catches catch-all and dead addresses before they show up as bounces in your metrics.
A list pulled six months ago and emailed today will have a 5-10% decay rate just from natural turnover. Re-verify before any send to a list older than 90 days.
What send behavior signals hurt email deliverability?
Receiving servers profile how you send, not just what you send. Patterns that look like a real human or a real business get rewarded; patterns that look automated and aggressive get filtered.
- Per-inbox volume caps: 30-50 cold emails per day per inbox is a safe ceiling for established Workspace/M365 inboxes. Pushing past this signals automation and damages reputation fast.
- Send time randomization: sequencers that send 50 emails between 9:00:00 and 9:00:30 get filtered. Real humans space sends out across hours.
- Reply tracking: when recipients reply, the reply has to actually arrive in the sending inbox. If replies bounce or get auto-archived, receiving servers learn the conversation is one-sided.
- Engagement signals: sustained low open rates (under 30% on a clean cold list) signal that mail is hitting spam. The fix is upstream: domain reputation, warmup, list quality, not new subject lines.
When should you focus on email content to improve deliverability?
After everything underneath is solid. Content optimization is the last layer, and it only moves the needle when the infrastructure, reputation, warmup, and list are all clean.
The content rules are simpler than most "spam trigger word" lists make them sound:
- Plain text or near-plain text outperforms image-heavy HTML for cold and follow-up email
- One clear call to action per email, not three
- Short paragraphs, conversational tone, no marketing copy in the first paragraph
- Avoid spam trigger phrases ("act now," "limited time," "100% free"), but the modern filters care less about specific words and more about engagement patterns
- Personalization at the open and a specific reason for the email, not a merge tag stuffed into a generic template
Content optimization at the top of a healthy stack moves reply rates from 3% to 6%. Content optimization at the top of a broken stack moves nothing because the email never arrives.
How do you monitor email deliverability on an ongoing basis?
Deliverability requires continuous monitoring, not a one-time audit. The signals that determine inbox placement shift constantly: new policy updates from Gmail and Microsoft, blocklists adding and removing domains, changes in your own list quality, drift in authentication records.
What to monitor weekly:
- Google Postmaster reputation across all sending domains
- Bounce rate and spam complaint rate per domain and per campaign
- Blocklist status (MultiRBL or equivalent)
- Inbox placement test results (Mail-Tester, GlockApps) for representative campaigns
- Reply rates and engagement, segmented by domain
A domain showing reputation drift over two consecutive weeks needs intervention before it becomes a blocked domain. The cost of catching it early is hours; the cost of recovering a flagged domain is weeks or a full domain replacement.
What is the most common mistake teams make when trying to fix deliverability?
They start with content tweaks, escalate to ESP changes, and only after months of degraded inbox placement realize the actual problem was authentication, infrastructure, or warmup. By that point, domain reputation is damaged enough that fixing the foundation takes weeks on its own.
The right order is authentication, then reputation, then infrastructure match, then warmup, then list hygiene, then send behavior, then content. Skip a layer at the bottom and nothing above it can compensate.
Frequently Asked Questions
How do I know if my emails are going to spam instead of the inbox?
Run an inbox placement test using Mail-Tester or GlockApps, which send a test message and report back on which folder it landed in across major providers. Also check Google Postmaster Tools for Gmail-specific spam rate data and Microsoft SNDS for Outlook. Sustained open rates below 20-25% on a cold list with a healthy domain are another signal that campaigns are hitting spam folders.
Can switching to a new domain fix my deliverability problems?
Yes, but only if you also fix the underlying problem that burned the old domain. A new domain with the same list hygiene issues, missing authentication, or warmup skipped will hit the same wall within 4-6 weeks. The new domain buys time; fixing the stack before you send is what actually solves it.
What is a safe daily sending volume per inbox for cold email?
30-50 cold emails per day per inbox is the safe ceiling for established Google Workspace or Microsoft 365 inboxes. Microsoft inboxes handle 5-10/day with warmup still running; Google inboxes can go 15-25/day once fully warmed. Go above these limits and receiving servers start treating the sending pattern as automated behavior, which triggers filtering.
Does email warmup need to run forever?
Yes, in the sense that you should always maintain a 2:1 warmup-to-cold ratio across your inbox pool throughout an active campaign. Warmup is not a one-time setup phase. It is a continuous reputation maintenance signal that keeps engagement data flowing to receiving servers alongside your cold sends.
What is the best way to improve email deliverability for cold outreach specifically?
Use secondary sending domains separate from your main business domain, set up real Google Workspace or Microsoft 365 inboxes (not SMTP relays), configure SPF, DKIM, and DMARC on every domain, complete a full 3-4 week warmup before sending any campaigns, and keep bounce rates under 2% with verified lists. The cold email deliverability guide covers each of these in detail.
At ScaledMail, we provision and manage the infrastructure layer end to end: secondary sending domains separate from your main business domain, real Google Workspace and Microsoft 365 inboxes, authentication configured correctly (SPF/DKIM/DMARC on every domain), IP rotation, continuous reputation monitoring. Warmup runs inside your sequencer (Smartlead, Instantly, EmailBison, PlusVibe), where the engagement signals live. If you want the foundation built right before the first email goes out, book a call or see the setup.
