UitbreidenOS Email Deliverability Skill
When to activate
- Open rates drop unexpectedly (> 20% decline week-over-week)
- A campaign lands in spam or promotions folder instead of inbox
- You're setting up a new sending domain and need to configure authentication
- You've never audited your sending infrastructure and aren't sure if it's configured correctly
- Launching a new email platform or IP address and need a warm-up plan
- You're seeing high bounce rates (> 2%) or spam complaint rates (> 0.1%)
When NOT to use
- Email copywriting — use
/email-sequenceor/email-campaignskills - Campaign strategy decisions — this skill is about infrastructure and hygiene, not messaging
- CRM data management — use your CRM tool; this skill diagnoses sending health
- One-off transactional emails you control end-to-end (password resets, receipts) — focus on marketing sends
Instructions
Full deliverability audit
Run a deliverability audit on my email sending setup.
My setup:
- Email platform: [Mailchimp / Klaviyo / HubSpot / SendGrid / Postmark / other]
- Sending domain: [e.g., newsletter.mycompany.com or mycompany.com]
- Monthly send volume: [X emails/month]
- List size: [X subscribers]
- List age: [how old is the oldest segment?]
- Average open rate (last 3 months): [X%]
- Average click rate: [X%]
- Bounce rate: [X%]
- Spam complaint rate: [X%] (find in your platform's analytics)
- Current inbox placement: [inbox / promotions / spam — or unknown]
Perform a diagnostic across these areas:
## 1. Authentication (SPF / DKIM / DMARC)
Check these records for [DOMAIN]:
SPF: verify the TXT record includes your sending platform's servers
DKIM: verify the CNAME or TXT records from your platform are active
DMARC: verify a DMARC policy exists and what it does (none / quarantine / reject)
What each means:
- SPF missing → easy spam classification, some providers reject outright
- DKIM missing → no cryptographic signature → treated as unsigned/unverified mail
- DMARC missing → domain spoofing trivial → providers penalise the domain
Recommended DMARC starting policy:
v=DMARC1; p=none; rua=mailto:dmarc-reports@yourdomain.com; pct=100
Move to p=quarantine after 30 days of clean reports, then p=reject after 60 days.
## 2. Sending domain configuration
- Are you sending from a subdomain (newsletter.company.com) or root domain?
Recommendation: subdomain for marketing, root domain for transactional — separate reputation pools
- Is the From address matching the authenticated domain?
- Is the Reply-To different from the From? (not a problem, but note it)
- Does the sending IP have reverse DNS (PTR record)?
## 3. Content analysis
Paste a recent email HTML + text version below and I'll scan for:
- Spam trigger words in subject line and body
- Text-to-image ratio (< 20% text = likely promotions folder)
- Link domains — are you using a custom click-tracking domain?
- Alt text on images (missing = spam signal)
- Unsubscribe link presence (legally required, improves deliverability)
- List-Unsubscribe header (must be present in headers)
- Physical address in footer (CAN-SPAM requirement)
## 4. List hygiene
Provide your list breakdown:
- Total subscribers: [X]
- Never opened in 90 days: [X] → suppression candidate
- Never opened in 180 days: [X] → sunset / re-engagement needed
- Hard bounces: [X] → must remove immediately
- Soft bounces (3+ times): [X] → remove
- Unsubscribes not honoured within 10 days: [X] → legal risk, fix immediately
## 5. Engagement segmentation
The most important deliverability factor in 2024+ is engagement.
Gmail and Apple Mail filter primarily based on whether recipients engage.
Segment your list:
- Highly engaged: opened or clicked in last 30 days → Priority 1 send
- Engaged: opened in last 90 days → Standard send
- Lightly engaged: last open 90-180 days → Re-engagement campaign before including
- Inactive: no open in 180+ days → Sunset sequence, then remove
Never send to inactive subscribers mixed with engaged subscribers.
The complaint and non-engagement rate from inactive segments penalises your entire domain reputation.
## 6. Deliverability score summary
| Area | Status | Action Required |
|---|---|---|
| SPF | ✓ / ✗ | [fix if missing] |
| DKIM | ✓ / ✗ | [fix if missing] |
| DMARC | ✓ / none / reject | [set policy] |
| Subdomain isolation | ✓ / ✗ | [split if needed] |
| List hygiene | Clean / Issues | [describe issues] |
| Engagement segments | Segmented / Unsegmented | [action] |
| Content flags | [N issues found] | [list] |
Overall health: Green / Amber / Red
Priority actions ranked by impact: [numbered list]
DNS record configuration guide
Generate the exact DNS records I need to configure for [SENDING PLATFORM] on domain [DOMAIN].
Platform: [Mailchimp / Klaviyo / SendGrid / Postmark / HubSpot / other]
Sending domain: [yourdomain.com or subdomain]
Current DNS provider: [Cloudflare / Route53 / GoDaddy / Namecheap / other]
Generate:
## SPF Record
Type: TXT
Host: @ (or subdomain)
Value: [platform-specific include statement]
Example: "v=spf1 include:sendgrid.net include:_spf.google.com ~all"
TTL: 3600
Note: only ONE SPF record per domain/subdomain. If you already have one, add the new include to it — do not create a second TXT record.
## DKIM Records
[Platform provides these — list the CNAME or TXT records with host and value]
Type: CNAME or TXT (platform-specific)
TTL: 3600
## DMARC Record
Type: TXT
Host: _dmarc.[domain]
Value: v=DMARC1; p=none; rua=mailto:dmarc@[domain]; pct=100
Start with p=none. Review reports for 30 days. Move to p=quarantine, then p=reject.
## BIMI Record (optional — brand logo in inbox)
Requires DMARC with p=quarantine or p=reject first.
Type: TXT
Host: default._bimi.[domain]
Value: v=BIMI1; l=https://[domain]/logo.svg; a=;
## Verification steps after DNS propagation (24-48 hours)
Test SPF: use MXToolbox SPF record checker
Test DKIM: send a test email and check headers in Gmail (View source)
Test DMARC: check [domain] at dmarcanalyzer.com
Test deliverability: send to mail-tester.com to get a score out of 10
Spam trigger word scanner
Scan this email for spam triggers.
Subject line: [paste]
Preview text: [paste]
Email body: [paste plain text or HTML]
Check for:
1. Classic spam words in subject (avoid entirely):
- Financial: "free money", "guaranteed income", "no risk", "earn $", "cash"
- Urgency abuse: "act now", "limited time!!!", "hurry", "don't miss out"
- Too promotional: "best price", "buy now", "discount", "lowest price"
- Phishing patterns: "click here", "verify your", "confirm your account"
- EXCESSIVE CAPS AND EXCLAMATION MARKS!!!
2. Body content issues:
- Image-to-text ratio: images with no alt text + minimal copy = promotion/spam
- Links to suspicious domains or unrelated tracking domains
- Missing or buried unsubscribe link
- No physical address in footer
3. Subject line length and punctuation:
- Optimal length: 30-50 characters
- Avoid: 3+ punctuation marks, 3+ emoji in a row
- Avoid: all lowercase or ALL CAPS subject lines
4. HTML issues:
- Inline styles only (external CSS can be stripped)
- Clean HTML — not copy-pasted from Word (Word embeds hidden tags)
- Text-only version present (HTML without plain-text backup = spam signal)
Output:
- Spam risk score: Low / Medium / High
- Specific triggers found and which rule they violate
- Revised subject line (if needed)
- Top 3 body fixes
New domain warm-up schedule
Build a warm-up schedule for a new sending domain or IP.
Domain/IP: [new sending domain or IP address]
Target send volume: [X emails/month at full scale]
Starting list quality: [verified opt-in, double opt-in, or imported/unknown]
Platform: [ESP name]
Warm-up principles:
1. Start with your most engaged subscribers (recent opens and clicks) — they signal positive engagement
2. Ramp slowly — doubling or tripling too fast triggers spam filters
3. Monitor bounce rate and complaint rate daily during warm-up
4. Never send to a cold/inactive list during warm-up — ruins the domain's reputation from day 1
5. Consistent daily sending beats irregular large sends
Warm-up schedule:
Week 1:
- Daily volume: 50 emails
- Send to: Most engaged subscribers (last 7 days)
- Bounce rate threshold: < 1%
- Complaint threshold: < 0.05%
Week 2:
- Daily volume: 200 emails
- Send to: Engaged (last 30 days)
- Thresholds: same
Week 3:
- Daily volume: 500 emails
- Send to: Engaged (last 60 days)
Week 4:
- Daily volume: 1,000-2,000 emails
- Send to: Engaged (last 90 days)
Month 2:
- Ramp to 10% of target volume
- Begin including moderately engaged (last 180 days)
Month 3+:
- Full volume, all verified subscribers
- Inactive > 180 days: sunset campaign before including
If bounce rate exceeds 2% or complaint rate exceeds 0.1% at any stage:
STOP the ramp. Diagnose. Clean the list. Resume from the previous volume tier.
Generate my specific weekly schedule from [START DATE] to reach [TARGET VOLUME] by [TARGET DATE].
List hygiene SOP
Generate a list hygiene standard operating procedure for [PLATFORM].
Current list: [X subscribers]
Current problems: [high bounces / low open rate / spam complaints / all of the above]
Hygiene checklist (run monthly):
1. Remove hard bounces immediately
Definition: email address doesn't exist or is permanently undeliverable
Action: automatically suppressed by most platforms — verify your platform does this
2. Remove soft bounce accumulation
Definition: 3+ soft bounces in 90 days (mailbox full, temporary server issue)
Action: move to suppression list, re-verify via an email verification service
3. Remove spam complainants
Definition: subscriber clicked "mark as spam" (reported to you via feedback loop)
Action: immediately suppress, do not resubscribe even if they ask nicely
4. Sunset inactive subscribers (quarterly)
Definition: no email open in 180 days
Process:
a. Send 3-email re-engagement campaign over 2 weeks
b. Track who opens or clicks — restore to active list
c. After 3 emails with no engagement: remove permanently
d. Do not resend to removed contacts — respect their implicit opt-out
5. Verify new list imports
Before sending to any imported list (trade show, purchased, old CRM data):
- Run through email verification service (NeverBounce, ZeroBounce, BriteVerify)
- Remove unknown/risky addresses (> 5% risky = don't send at all)
- Double opt-in confirmation before adding to marketing sends
Output: monthly hygiene calendar and template email for re-engagement sequence.
Key deliverability metrics reference
interface DeliverabilityHealth {
// Inbox placement rate (use GlockApps, Litmus, or 250ok to measure)
inboxPlacementRate: number // target: > 90%
// From your ESP dashboard
openRate: number // target: > 20% (Gmail/Apple Mail clip post-privacy)
clickRate: number // target: > 2%
bounceRate: number // alert at > 2%, danger at > 5%
spamComplaintRate: number // alert at > 0.08%, danger at > 0.1% (Google threshold)
unsubscribeRate: number // alert at > 0.5% per campaign
// List health
activeEngagedPercent: number // subscribers who opened in last 90 days / total
inactivePercent: number // no open in 180 days / total
}
const DELIVERABILITY_THRESHOLDS = {
inboxPlacementRate: { healthy: 0.90, concern: 0.80, critical: 0.70 },
bounceRate: { healthy: 0.02, concern: 0.05, critical: 0.10 },
spamComplaintRate: { healthy: 0.0005, concern: 0.001, critical: 0.003 },
// Note: Google's postmaster tools flag domains at 0.1% complaint rate
// Apple Mail Privacy Protection (iOS 15+) inflates open rates — do not use open rate alone
}
Example
User: My open rates dropped from 32% to 19% over 3 months. I haven't changed anything. What do I check first?
Diagnosis framework:
A 13-point open rate drop in 3 months with no content changes points to one of these causes
in order of likelihood:
1. List decay — inactive subscribers accumulated and are dragging engagement
Check: % of your list with no open in 90 days. If > 30%, this is your culprit.
Fix: suppress inactive segment immediately, run re-engagement campaign
2. Deliverability shift — domain or IP reputation changed (common after a high-bounce send)
Check: log into Google Postmaster Tools for your domain. Look at domain reputation and
spam rate charts. A drop in domain reputation directly correlates with inbox placement loss.
Fix: run full authentication audit, clean list, reduce send frequency for 30 days
3. Apple Mail Privacy Protection accumulation — if your list is heavily iOS users,
artificially inflated opens from 2021 are now churning out as those users become inactive.
This isn't a real deliverability problem — it's a measurement problem.
Fix: switch to click rate as primary engagement metric. Open rate is unreliable for iOS.
4. Sending domain change — did you migrate to a new subdomain, ESP, or IP without re-warming?
Check: email headers of a sent campaign. What's the actual sending IP?
Fix: warm-up schedule for new infrastructure.
Start with Google Postmaster Tools — it's free and tells you within 24 hours whether Gmail is
classifying your domain as spammy. That narrows the diagnosis immediately.