Emails Going to Spam? 10 Rules to Improve Your Email Deliverability [Tips]

You've set up your email correctly. So why are your emails still going to spam?

The answer is almost never just one thing. Email deliverability is a combination of technical setup, sender reputation, content quality, and sending behavior. You need all of these working together.

Below are 10 rules that actually move the needle — used by the Vanderbuild team when setting up outbound campaigns for SaaS and tech companies.

Rule 1: Always Use a Separate Domain for Cold Outreach

Never run cold email from your main company domain. Period.

If you send cold email from yourcompany.com and something goes wrong — spam complaints, poor engagement, inbox placement issues — you risk damaging the domain that powers all your marketing, customer comms, and sales follow-ups.

Instead, register a lookalike domain:

• yourcompany.co, yourcompany.io, yourbrand.net, try-yourproduct.com

Configure it properly with SPF, DKIM, and DMARC, warm it up, and keep it separate from your main operations.

Rule 2: Set Up SPF, DKIM, and DMARC Correctly

These three DNS records are the foundation of email authentication. Without them, inbox providers have no reason to trust you.

• SPF — Specifies which servers are allowed to send email on behalf of your domain
• DKIM — Adds a cryptographic signature to each email, confirming it hasn't been tampered with
• DMARC — Defines what to do when SPF or DKIM checks fail, and gives you reporting visibility

Most cold email tools will walk you through the setup. The most common mistake is setting DMARC to none and never revisiting it. At minimum, move to quarantine once you're confident in your authentication setup.

Rule 3: Warm Up Every New Inbox Before Sending

Warming up is not optional. It's the process of gradually increasing sending volume on a new inbox or domain, while exchanging legitimate emails with real (or simulated) recipients.

Inbox providers track the history of your sending behavior. A brand-new domain that suddenly sends 500 emails in a day looks exactly like a spam operation — because it usually is.

The minimum warm-up period before any cold sending: 3 to 4 weeks.

Tools that handle this:

• Warmup Inbox
• Mailreach
• Built-in warm-up in Instantly Inbox Placement or Smartlead

Keep warm-up running even after you start sending campaigns.

Rule 4: Don't Send More Than 50 Emails Per Inbox Per Day

This is the most commonly broken rule. People want volume, so they push 200+ emails per inbox and wonder why deliverability collapses.

The safe ceiling is 30 to 50 emails per inbox per day, depending on the age of the inbox and how established your sending reputation is.

If you need to send more, add more inboxes and domains — don't just increase per-inbox volume.

Simple math: if you want to reach 3,000 prospects per month with a 3-step sequence, you need around 10 inboxes running in parallel.

Rule 5: Keep Your Email Copy Clean

Spam filters analyze content. Certain patterns consistently trigger filters:

• Words like "free," "guaranteed," "no risk," "limited time offer"
• All-caps subject lines or excessive punctuation (!!!, ???)
• Heavy HTML formatting, tracking pixels, or multiple links in the first email
• Attachments in a cold email (never do this)

The safest format for cold email: plain text, or very light HTML. Write like you're sending from your personal email account — because that's what inbox providers compare it to.

Rule 6: Personalize Enough That It Doesn't Look Like a Blast

Generic emails not only perform poorly — they also signal to filters that this is mass outreach.

You don't need a novel of personalization. One or two specific, accurate details per email are enough to differentiate from a template blast:

• A reference to a recent hire, product launch, or LinkedIn post
• A mention of the company's tech stack, geographic market, or funding stage
• A specific framing of your value proposition to their situation

Tools like Clay make it possible to generate these personalizations at scale without writing each one manually.

Rule 7: Monitor Your Deliverability Actively

Most teams set up a campaign and check reply rates. That's not enough.

You should also be tracking:

• Spam placement rate — What percentage of your emails end up in spam across major providers (Gmail, Outlook, etc.)
• Bounce rate — Anything above 3–5% is a warning sign
• Unsubscribe and complaint signals — High complaint rates damage your sender reputation fast

Tools for monitoring:

• Warmup Inbox, Instantly Inbox Placement or MXToolbox for DNS/blacklist checks
• Google Postmaster Tools — free, essential if you're emailing Gmail accounts

Rule 8: Verify Your Email List Before Sending

Bounced emails are one of the fastest ways to damage your domain's reputation. Every hard bounce signals to inbox providers that you're not maintaining your list properly.

Before any campaign:

• Run your list through an email verification tool
• Remove any addresses flagged as invalid, risky, or catch-all
• Never buy email lists from third parties — they are almost always partially invalid and will tank your bounce rate

Email verification tools worth using:

• Findymail
• Prospeo
• ZeroBounce
• NeverBounce

Rule 9: Space Out Your Follow-Ups

Multi-step sequences are standard in cold outreach. But the timing matters more than most people think.

Sending follow-ups too quickly — every day or two — increases the chance of complaints and creates a pattern that filters recognize as aggressive automation.

The general rule:

• 3–4 days between the first and second email
• 5–7 days between subsequent follow-ups
• No more than 4–5 total touches in a sequence

Also: make sure your platform automatically stops sending when someone replies. Sending a follow-up after receiving a reply is one of the easiest ways to create a spam complaint.

Rule 10: Rotate Sending Across Inboxes and Domains

Don't send all your campaign volume from one inbox or one domain. Spread the load.

This means:

• Rotating which inbox sends to which prospect, so no single inbox spikes in volume
• Keeping 2–3 sending domains per campaign when possible
• Monitoring individual inbox reputation, not just campaign-level metrics

Most modern cold email tools — Instantly, Smartlead, Lemlist — support inbox rotation natively.

Summary

Deliverability isn't a one-time setup. It's an ongoing process of maintaining your technical infrastructure, monitoring your sender reputation, and adjusting based on data.

If you're currently landing in spam, start with the basics: authentication, warm-up, and list quality. Most deliverability problems trace back to one of those three.

And if you're setting up outbound from scratch and want to avoid the most common mistakes, we've put together a step-by-step guide: How to Create an Effective Cold Email Campaign.