Email Deliverability 101 for Agencies: Keep Your Cold Emails Out of Spam

Introduction

You can write the best cold emails in the world—if they don’t reach the inbox, none of it matters. Deliverability isn’t mysterious. It’s a set of technical basics plus behavior patterns ISPs reward.

This guide translates the technical bits into plain English, with concrete steps, examples, and benchmarks agencies can follow.

Deliverability vs. Delivery: The Difference

Delivery is “accepted by the server.” Deliverability is “landed in the primary inbox.” Aim for the latter; promotions or spam tabs crush reply rates.

What ISPs Actually Score

ISPs weigh authentication, historical engagement, complaint rate, bounce rate, content fingerprints, and sending behavior (timing, spikes, diversity).

Technical Setup Checklist

Own your sending domain or subdomain, set SPF/DKIM/DMARC, use a custom tracking domain, and align sender identities. Document this in a repeatable SOP.

WarmOpener Setup (Quick)

Connect mailboxes and verify domain auth
Add custom tracking domain (e.g., t.company.com)
Enable per-inbox throttling and rotation

SPF: Allow Only Who Should Send

Publish a minimal SPF record listing approved senders. Avoid +all or overly long chains. Keep under 10 DNS lookups.

v=spf1 include:_spf.google.com include:send.example.com -all

DKIM: Sign Every Message

Enable DKIM signing for your sending service. Rotate keys annually. Use 2048-bit keys where supported. Signed mail earns trust.

google._domainkey  IN  TXT  "v=DKIM1; k=rsa; p=..."

DMARC: Set Policy by Stage

Start with p=none to monitor. Move to quarantine once issues are fixed. Graduate to reject when confident. Report addresses help catch misconfigs.

_dmarc  IN  TXT  "v=DMARC1; p=none; rua=mailto:dmarc@company.com; fo=1"

Custom Tracking Domains

Don’t use shared tracking links. Set a branded tracking domain (e.g., t.company.com) to prevent shared-reputation penalties.

Choosing The Right From Name

Use a human name plus brand ("Maya from BrightAgency"). Keep consistent across touches. Avoid no-reply addresses.

Plain Text vs. HTML in Cold

Plain text or very light HTML wins for cold. Avoid heavy images, complex layouts, and trackers that look like marketing blasts.

Safe Template (Cold)

Subject: Quick question about {{trigger}}

Hi {{first_name}},

{{personalized_intro}}

We recently helped {{peer_company}} with {{specific_outcome}}.
Open to a quick 10-min compare notes?

Best,
{{your_name}}

The Role of Link Count

One link or none in the first email is safest. Multiple links raise suspicion, especially to domains with poor reputation.

Subject Line Risk Factors

All caps, excessive punctuation, exaggerated claims, and bait-and-switch subjects increase spam probability. Be specific and sincere.

Body Language That Triggers Filters

Over-promising, spammy phrases ("act now"), and templated patterns harm placement. Conversational, context-rich language performs best.

Image Usage and Ratios

If you must add an image (rare in cold), keep text-to-image ratio high and ensure images are compressed and hosted on reputable domains.

Signature Do’s and Don’ts

Include your real name, role, and company. Avoid multiple logos or banners. Keep links minimal; no tracking on signature links if possible.

—
Maya Patel | Founder, BrightAgency
brightagency.com

Variable Personalization Lowers Spam

Personalized intros, references to recent events, or page-level context produce unique bodies per email—reducing duplicate-content detection.

WarmOpener Workflow: Unique Bodies at Scale

Upload unlimited columns (e.g., recent_news, pricing_note, blog_quote). WarmOpener uses your OpenAI API key to generate unique intros and value props for each prospect.

Avoiding Duplicate Content

Ship variants for each segment. WarmOpener can auto-generate safe variations while preserving your messaging intent.

List Source and Consent Risk

Never purchase lists. Verify addresses. Use role accounts cautiously. Maintain a suppression list and honor opt-outs quickly.

Inbox Warming Basics

Ramp volume over weeks. Mix in replies and human conversations. WarmOpener respects warm-up limits with controllable throttles.

Seed Testing Reality Check

Seed inboxes offer directional hints, not absolutes. Focus on trends across real sends and engagement metrics, not one-off seed results.

Postmaster Tools to Watch

Use Google Postmaster (for Gmail) to monitor domain and IP reputation, delivery errors, and spam rates. Investigate dips fast.

Handling Bounces the Right Way

Hard bounces: suppress immediately. Soft bounces: retry later with backoff; suppress after repeated failures. Keep bounce rate under 2%.

Bounce SOP (Quick)

Verify lists before sending
Suppress hard bounces instantly
Retry soft bounces up to 2–3 times

Spam Complaints and Feedback Loops

Zero is the goal. A few complaints happen; suppress complainers permanently. If a spike occurs, pause and review segment quality and content.

Throttling and Scheduling

Spread sends across hours and days. Avoid bursts. Randomize send intervals. WarmOpener automates safe pacing across multiple inboxes.

Example: 200/day per inbox
→ 25/hour over 8 hours
→ ±15% random jitter per send

Engagement Shaping in Sequences

Follow-ups should add value, not pressure. Asking a new question or sharing a relevant resource improves engagement and reputation.

Cold vs. Warm vs. Transactional

Use distinct domains or subdomains for cold vs. product or customer emails. Don’t let cold outreach reputation taint transactional mail.

Mailbox and Domain Age Matters

New domains and mailboxes face stricter scrutiny. Start smaller, aim for high engagement, and expand only when metrics are healthy.

Shared vs. Dedicated IP

Most agency programs use shared IPs via inbox providers. Dedicated IPs make sense only at very high, steady volumes with strong reputation.

Role Accounts and Traps

Avoid info@, sales@, admin@ where possible—often higher risk. Spam traps hide in scraped or purchased lists; verification is essential.

File Attachments and Alternatives

Skip attachments in cold emails. Use a short, single link to a Google Doc or Loom when necessary; ensure the destination domain has good reputation.

Link Domains and UTM Best Practices

Use branded, clean URLs. Keep UTM parameters simple. Avoid URL shorteners. Test the landing page for speed and security (HTTPS).

Reply Handling and Threading

Reply behavior (fast, helpful, natural) strengthens reputation. Keep follow-ups within the same thread when appropriate to signal an ongoing conversation.

Dealing with Soft Blocks

If Gmail temp-limits sends, pause, lower volume, and vary content. Check authentication and recent complaint trends before resuming.

Recovery Plan After a Spam Spike

Pause campaigns. 2) Fix root cause (lists/content). 3) Lower volume 50–70%. 4) Send to engaged segments first. 5) Gradually restore normal volume.

Template to reset tone (follow-up):

Subject: Closing the loop

Hi {{first_name}},

Dropping this in case timing wasn’t right earlier. I put a
2-slide snapshot together specific to {{company}}—happy to send
if useful. Otherwise, I’ll close the loop here.

Best,
{{your_name}}

Agency Playbook Template

Create a one-page deliverability SOP: setup steps, daily checks, bounce thresholds, complaint thresholds, pause rules, and recovery steps. Train every sender.

How WarmOpener Safeguards Deliverability

WarmOpener uses rotation, throttling, and per-inbox pacing; supports custom tracking domains; and encourages unique, AI-personalized content using your own OpenAI API key—so you can scale without sacrificing inbox placement.

Conclusion

Deliverability is the compound interest of cold email. Get the basics right, protect your reputation, and your replies rise month after month.

Try WarmOpener free and see how safe scaling meets authentic personalization.