manus-skills/.kortix/opencode/skills/content/email-best-practices/references/deliverability.md

Email Deliverability

Maximizing the chances that your emails are delivered successfully to the recipients.

Email Authentication

Required by Gmail/Yahoo/Microsoft - unauthenticated emails will be rejected or spam-filtered.

SPF (Sender Policy Framework)

Specifies which servers can send email for your domain.

v=spf1 include:amazonses.com ~all

• Add TXT record to DNS
• Use ~all (soft fail)

DKIM (DomainKeys Identified Mail)

Cryptographic signature proving email authenticity.

• Your email service will provide you with a TXT record

DMARC

Policy for handling SPF/DKIM failures + reporting.

v=DMARC1; p=none; rua=mailto:dmarc@example.com

Rollout: p=none (monitor) → p=quarantine; pct=25 → p=reject

Learn more: https://resend.com/blog/dmarc-policy-modes

Verify Your Setup

Check DNS records directly:

# SPF record
dig TXT example.com +short

# DKIM record (replace 'resend' with your selector)
dig TXT resend._domainkey.example.com +short

# DMARC record
dig TXT _dmarc.example.com +short

Expected output: Each command should return your configured record. No output = record missing.

Sender Reputation

IP Warming

New IP/domain? Gradually increase volume:

Week	Daily Volume
1	50-100
2	200-500
3	1,000-2,000
4	5,000-10,000

Start with engaged users. Send consistently. Don't rush.

Learn more: https://resend.com/docs/knowledge-base/warming-up

Maintaining Reputation

Do: Send to engaged users, keep bounce <4%, complaints <0.1%, remove inactive subscribers.

Don't: Send to purchased lists, ignore bounces/complaints, send inconsistent volumes

Bounce Handling

Type	Cause	Action
Hard bounce	Permanent failure to deliver	Remove immediately
Soft bounce	Transient failure to deliver	Retry: 1h → 4h → 24h, remove after 3-5 failures

Targets: <1% good, 1-3% acceptable, 3-4% concerning, >4% critical

Complaint Handling

Targets: <0.01% excellent, 0.01-0.05% good, >0.05% critical

Reduce complaints:

• Only send to opted-in users
• Make unsubscribe easy and immediate
• Use clear sender names and "From" addresses

Feedback loops: Set up with Gmail (Postmaster Tools), Yahoo, Microsoft SNDS. Remove complainers immediately.

Infrastructure

Dedicated sending domain: Use different subdomains for different sending purposes (e.g., t.example.com for transactional emails and m.example.com for marketing emails).

DNS TTL: Low (300s) during setup, high (3600s+) after stable.

Troubleshooting

Emails going to spam? Check in order:

1. Authentication (SPF, DKIM, DMARC)
2. List-Unsubscribe header — required by Gmail/Yahoo since Feb 2024 (see Compliance)
3. Sender reputation (blacklists, complaint rates)
4. Content
5. Sending patterns (sudden volume spikes)

Diagnostic tools:

• Google Postmaster Tools - Domain reputation and spam rates
• mail-tester.com - Send a test email, get deliverability score
• MXToolbox - Check blacklist status

Related

• List Management - Handle bounces and complaints to protect reputation
• Sending Reliability - Retry logic and error handling