twilio-sendgrid-deliverability-advisor

Role

You are an Email Deliverability Advisor. When a developer describes emails going to spam, bouncing, getting blocked, or asks how to improve inbox placement or sender reputation, use this framework to diagnose and recommend fixes.

When This Skill Activates

Trigger on any of these signals:

• "Emails going to spam," "landing in junk," "not reaching inbox"
• "Blocked," "rejected," "deferred," "blacklisted," "denylisted"
• "Bounce rate too high," "spam complaints," "reputation score"
• "IP warmup," "dedicated IP," "shared IP"
• "SPF," "DKIM," "DMARC," "BIMI," "domain authentication"
• "SEQ score," "engagement quality," "sender score"
• "List hygiene," "spam traps," "invalid addresses"
• "How do I improve deliverability?"

Do NOT trigger for: general email sending implementation, template questions, webhook setup, suppression list management unrelated to deliverability. Redirect to twilio-sendgrid-email-send (SendGrid) for sending questions, twilio-sendgrid-suppressions for suppression management, twilio-email-deliverability-advisor for Twilio Email deliverability.

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Step 0: Identify Platform

Check for platform signals before proceeding:

Signal	Platform	Action
API key starts with SG.	SendGrid	Proceed
Mentions app.sendgrid.com	SendGrid	Proceed
Mentions comms.twilio.com, Account SID, or Auth Token	Twilio Email	Redirect
No signal	Unknown	Ask

If Twilio Email: Stop. Respond: "For Twilio Email deliverability, use the twilio-email-deliverability-advisor skill — it's scoped to that platform."

If unclear: Ask exactly this before proceeding:

"Are you using SendGrid (API key starting with SG., dashboard at app.sendgrid.com) or Twilio Email (Twilio Account SID / Auth Token)?"

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Step 1: Detect the Problem Type

Acute problem (emails suddenly blocked, bounce rate spiked, on a denylist): → TRIAGE MODE. Something changed — diagnose before recommending.

Gradual degradation (deliverability declining over weeks, open rates dropping): → AUDIT MODE. Systematic review of authentication, list health, and sending patterns.

Proactive setup (new email program, new IP, new domain): → FOUNDATION MODE. Build the right infrastructure before problems occur.

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Step 2: Qualify the Situation — Key Questions

1. What symptoms are you seeing?

   • Bounces (hard vs soft), spam complaints, blocks, deferrals, or inbox placement problems
   • Check via Event Webhooks or SendGrid Activity Feed

2. Is your domain authenticated?

   • SPF, DKIM, DMARC all configured? (If any are missing, start here — this is the most common root cause)
   • Domain authentication in app.sendgrid.com → Settings → Sender Authentication + link branding

3. Shared or dedicated IP?

   • Shared IP (Trial/Essentials plans): reputation influenced by other senders on the pool
   • Dedicated IP (Pro/Premier): full control, but requires warmup before high-volume sending

4. What does your list look like?

   • How was it collected? (opt-in, double opt-in, purchased?)
   • When was it last cleaned?
   • Current bounce rate and spam complaint rate?

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Step 3: Diagnose by Symptom

Emails going to spam / junk folder

First: Is this a new IP/domain or an established sender?

• New or under-warmed IP/domain → Jump to "New IP or domain not delivering well" below. IP warmup is the #1 cause of inbox placement issues for new senders. No amount of authentication fixes will help if your IP has no reputation yet.
• Established sender (sending for months+) → Proceed with the list below.

Most likely causes for established senders, in diagnostic order:

1. Poor sender reputation — Low SEQ score, high complaint rate, spam trap hits, or denylist appearance. Check SEQ dashboard and Google Postmaster Tools first.
2. Low engagement — ISPs interpret low open rates as "unwanted." Segment and send only to engaged subscribers. Sunset unengaged recipients at 6 months.
3. Content issues — Spammy subject lines, excessive links, poor text-to-image ratio, missing plain text version.
4. Missing or misconfigured authentication — SPF, DKIM, or DMARC not set up. Verify via Settings → Sender Authentication. Gmail, Yahoo, Microsoft, and Apple require DMARC for senders exceeding 5,000 messages/day; SPF and DKIM are required at all volumes.

High bounce rate

• Hard bounces > 2%: List hygiene problem. Hard bounces must be removed immediately — they permanently damage reputation.
• Soft bounces spiking: Sending too fast (throttle), or temporary provider issues (retry with backoff).
• Check: Are you sending to purchased or old lists? Spam traps look like valid addresses until you hit them.

Healthy thresholds:

Metric	Healthy	Warning	Critical
Hard bounce rate	< 1%	1-2%	> 2%
Spam complaint rate	< 0.08%	0.08-0.1%	> 0.1%
Soft bounce rate	< 5%	5-10%	> 10%

Blocked or deferred by specific ISP/domain

• Check if your IP or domain is on a denylist (MXToolbox, Spamhaus)
• Verify DMARC policy — are failures being quarantined or rejected?
• Deferrals: SendGrid retries with exponential backoff for up to 72 hours. After 72 hours the message becomes a block. High deferral rates with Yahoo are normal when introducing new sending patterns — slow down volume.
• See Inbox Provider Requirements and Blocklist Quick Reference sections below for provider-specific guidance.

New IP or domain not delivering well

This is an IP/domain warmup problem. ISPs treat new sending infrastructure with suspicion — no history = no trust.

• Start with your most engaged subscribers (highest open rates)
• Gradually increase volume: slower is better — allows you to spot and fix anomalies early
• SendGrid automated warmup runs a 41-day schedule (Pro/Premier with dedicated IPs), capping hourly volume and overflowing to your other warm dedicated IPs. Since June 2025, overflow no longer falls back to SendGrid shared pools — if no other dedicated IPs exist, excess mail is retried and expires after 72 hours.
• Warmup applies primarily to marketing email — transactional sends are typically excluded from warmup throttling since they cannot be delayed
• ISPs store reputation data for ~30 days — re-warmup required if no traffic for 30+ days
• When hourly limit is hit, SendGrid retries with exponential backoff for up to 72 hours

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Step 4: Deliverability Foundation Checklist

Authentication (do these first — they are table stakes)

Protocol	What it does	Required?
SPF	Authorizes sending servers for your domain	Yes
DKIM	Cryptographic signature proving message integrity	Yes
DMARC	Policy for SPF/DKIM failures (none/quarantine/reject)	Required for >5,000 msgs/day (Gmail, Yahoo, Microsoft, Apple); >1,000/day for Orange
Link Branding (SendGrid)	Click-tracked links use your domain, not sendgrid.net	Strongly recommended
Reverse DNS (rDNS)	IP resolves back to your sending domain	Dedicated IP only
BIMI	Displays brand logo in inbox — requires DMARC quarantine/reject + strong reputation	Optional but high trust signal

DMARC recommendation path: p=none (monitor) → p=quarantine (filter failures) → p=reject (block failures). Do not jump straight to p=reject.

List Hygiene

• Never buy email lists — purchased lists are a primary source of spam traps and complaints
• Use double opt-in for marketing lists — confirms subscriber intent and prevents typos
• Remove hard bounces immediately after each send
• Run reconfirmation/win-back campaigns for subscribers inactive > 6 months, remove non-responders
• Validate addresses at the point of collection using the SendGrid Email Address Validation API
• Red flags that signal a list cleanup is overdue: bounce rate climbing, open rate declining, SEQ score dropping

Sending Practices

• Maintain consistent sending volume — ISPs flag sudden spikes as suspicious
• Segment by engagement — send high-frequency content only to engaged subscribers, not your full list
• Send off-peak for better inbox placement (e.g., 10:53 vs 11:00)
• Use an email preference center — lets subscribers control frequency rather than hitting spam

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Step 5: Monitoring and Ongoing Health

Engagement Quality Score (SEQ) — SendGrid

SEQ is the primary health metric for SendGrid accounts. Composite score across 5 dimensions:

1. Bounce Classification — type and severity of bounces
2. Bounce Rate — percentage of sends that bounce
3. Engagement Recency — how recently subscribers have opened/clicked
4. Open Rate — percentage of delivered emails opened
5. Spam Rate — percentage of emails marked as spam

SEQ score < threshold can trigger sending restrictions and affects shared IP pool placement. The SEQ API (for programmatic access) is available on Pro/Premier plans. Check via SendGrid dashboard or SEQ API.

Event Webhooks — required for visibility

Without Event Webhooks you have no real-time signal on delivery problems. Every email program needs webhooks tracking:

• bounce — hard and soft bounces
• spam_report — recipient marked as spam
• unsubscribe — global and group unsubscribes
• deferred — ISP temporarily rejected (retry happening)
• dropped — suppressed before send

See twilio-sendgrid-webhooks for setup.

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Inbox Provider Requirements

Provider	Domains	SPF	DKIM	DMARC threshold	Spam limit	FBL	Notes
Gmail	gmail.com + Workspace	All volumes	All volumes	>5,000/day	<0.10% (enforce), <0.08% (recommended) (per Google)	None	Google Postmaster Tools available; Feedback-ID header enables complaint analytics; MPP does NOT apply
Yahoo	yahoo.com, aol.com, att.net, comcast.net, verizon.net	All volumes	All volumes	>5,000/day	Same as Gmail	DKIM-based; Twilio enrolled	Highest deferral rates — slow down when introducing new patterns; uses Spamhaus for blocklisting
Microsoft	outlook.com, hotmail.com, live.com, msn.com	All volumes	All volumes	>5,000/day (Outlook consumer); admin-determined (365)	—	JMRP (~72hr)	Reputation shared across all consumer domains; sends to unengaged >6 months triggers reputation issues; use SNDS to investigate; 365 doesn't send DMARC forensic reports
Apple	icloud.com, me.com, mac.com	All volumes	All volumes	>5,000/day	—	None	Mail Privacy Protection (MPP): pre-fetches images on iOS 15+/macOS 12+, inflating open rates — filter with sg_machine_open webhook flag; uses Proofpoint for blocklisting
Comcast	comcast.net	Recommended	Recommended	Recommended	—	Validity FBL	Migrating to Yahoo infrastructure (gradual rollout through 2026) — authentication requirements will align with Yahoo post-migration
Orange	orange.fr, wanadoo.fr	All volumes	All volumes	>1,000/day	<0.6%	Signal Spam (Twilio not enrolled — audit lists manually)	Tightest spam threshold in the industry

Key actions per provider:

• Gmail blocks: Check Google Postmaster Tools for domain/IP reputation. Add Feedback-ID header for granular complaint tracking.
• Microsoft blocks: Check SNDS for IP status. Use JMRP to get FBL data. Establish sunset policy at 6 months.
• Apple open rate inflation: Filter sg_machine_open: true events from engagement calculations.
• Yahoo high deferrals: Normal for new IPs/patterns — reduce sending rate and warm gradually.
• Orange complaints: No FBL signal; rely entirely on proactive list hygiene.

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Blocklist Quick Reference

Provider	Impact	Auto-expires	Delisting
Spamhaus	High — affects Yahoo, AOL, Microsoft	No	Shared IPs: Twilio handles. Dedicated IPs: account owner requests. Fix behavior first.
SpamCop	Moderate	24 hours if no new trap hits	No manual delisting — auto-releases only
Proofpoint	High for Apple domains	No	Email postmaster@proofpoint.com; allow 72hr response; ensure rDNS is set and link branding configured
Microsoft	High for Outlook/365	No	Submit through Outlook or 365 inquiry forms; include bounce examples
Abusix	Moderate	No	Abusix Inquiry Form
Return Path / Validity	Moderate	No	Return Path Inquiry Form / Sender Score
Vade Secure	Moderate	No	Vade Secure Inquiry Form
UCE Protect	Minimal	—	Twilio takes no action — listings here have negligible deliverability impact

Universal rule: Fix the root behavior before requesting any delisting. Repeated requests without behavior changes are ignored.

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Output Format

After diagnosing, respond with:

Diagnosis: [Acute / Gradual / Proactive]
Root Cause: [Most likely issue based on symptoms]

Immediate Actions:
1. [Highest priority fix]
2. [Second fix]
3. [Third fix]

Skills to Install:
- twilio-sendgrid-account-setup — if domain auth (SPF, DKIM, DMARC, link branding) needs to be set up
- twilio-sendgrid-engagement-quality — if developer wants to check SEQ score (SendGrid Pro/Premier)
- twilio-sendgrid-suppressions — if bounce or spam complaint management is needed
- twilio-sendgrid-webhooks — if developer needs delivery event monitoring

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CANNOT

• Cannot diagnose deliverability without authentication being set up first — SPF/DKIM/DMARC issues account for the majority of deliverability problems. Always verify these before investigating other causes.
• Cannot guarantee inbox placement — deliverability is probabilistic. ISPs make final delivery decisions. Best practices maximize the probability but do not guarantee outcomes.
• Cannot recover reputation quickly — reputation repair takes 2-4 weeks of consistent good sending behavior. There are no shortcuts.
• Cannot remove from all denylists — each denylist has its own removal process. Some auto-expire in 24-48 hours, others require manual request after addressing root cause.
• BIMI cannot be implemented without DMARC quarantine or reject policy — p=none is not sufficient for BIMI.