email-deliverability

Files (6)

• README.md
• SKILL.md
• evals.json
• bounce-handling.md · references/
• spf-dkim-dmarc.md · references/
• warm-up-and-reputation.md · references/

SKILL.md (first lines)

## Key principles

1. **Authenticate everything, no exceptions** - Every sending domain must have
   SPF, DKIM, and DMARC configured. Missing any one of these is enough for
   major mailbox providers (Gmail, Outlook) to treat your mail as suspicious.
   Authentication is table stakes, not a nice-to-have.

2. **Reputation is earned slowly and lost instantly** - Sender reputation is
   built over weeks of consistent, low-complaint sending. A single spam trap
   hit or complaint spike can tank your reputation overnight. Treat every send
   decision as a reputation decision.

3. **Bounces are signals, not noise** - Every bounce carries information about
   your list health, infrastructure, or content. Hard bounces must be removed
   immediately. Soft bounces must be tracked and acted on. Ignoring bounces
   is the fastest path to blocklisting.

4. **Warm up before you scale up** - New IPs and domains have zero reputation.
   Mailbox providers throttle unknown senders aggressively. A proper warm-up
   plan ramps volume gradually over 2-6 weeks, proving you are a legitimate
   sender before asking for high throughput.

5. **Monitor continuously, not reactively** - By the time users report "emails
   aren't arriving," the damage is done. Monitor bounce rates, complaint rates,
   and inbox placement proactively. Set alerts on thresholds, not symptoms.

---

## Core concepts

Email deliverability is governed by three layers: **authentication** (proving
you are who you claim to be), **reputation** (proving you send mail people
want), and **behavior** (proving your sending patterns are consistent and
trustworthy).

**Authentication** uses three DNS-based protocols that work together. SPF
declares which IPs may send on behalf of your domain. DKIM attaches a
cryptographic signature to each message that receivers verify against a public
key in your DNS. DMARC ties SPF and DKIM together with a policy that tells
receivers what to do when authentication fails - and sends you reports about it.

**Reputation** is a score maintained by each mailbox provider independently.
It is influenced by complaint rates (users clicking "spam"), bounce rates,
spam trap hits, engagement signals (opens, clicks, replies), and sending
volume consistency. There is no single universal reputation score - Gmail,
Outlook, and Yahoo each maintain their own.

**Behavior** covers sending patterns: volume consistency, warm-up adherence,
list hygiene practices, and how you handle bounces and unsubscribes. Sudden
volume spikes, sending to stale lists, or ignoring unsubscribe requests all
signal spammer behavior to mailbox providers.

---

## Common tasks

### Configure SPF

SPF (Sender Policy Framework) declares which mail servers may send email for
your domain via a DNS TXT record.

```dns
example.com.  IN  TXT  "v=spf1 include:_spf.google.com include:sendgrid.net ip4:203.0.113.5 -all"
```

**Rules:**
- Only one SPF record per domain (multiple records cause permerror)
- Use `include:` for ESPs, `ip4:`/`ip6:` for your own servers
- End with `-all` (hard fail) for production, `~all` (soft fail) only during testing
- Stay under 10 DNS lookups total (each `include:` and `a:` costs one lookup)
- Use SPF flattening tools if you hit the 10-lookup limit

> The 10-lookup limit is the most common SPF misconfiguration. Each `include:`
> triggers recursive lookups. Monitor with `dig TXT example.com` and count.

### Configure DKIM

DKIM (DomainKeys Identified Mail) signs outgoing messages with a private key.
Receivers verify the signature against a public key published in DNS.

```dns
selector1._domainkey.example.com.  IN  TXT  "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBA..."
```

**Setup checklist:**
- Generate a 2048-bit RSA key pair (1024-bit is deprecated)
- Publish the public key as a TXT record at `<selector>._domainkey.<domain>`
- Configure your mail server or ESP to sign outgoing mail with the private key
- Use a unique selector per ESP so you can rotate keys independently
- Test with `dig TXT selector._domainkey.example.com` to verify publication
- Rotate keys annually - publish new key, wait 48h for propagation, then switch

> If your TXT record exceeds 255 characters, split it into multiple strings
> within a single TXT record. Most DNS providers handle this automatically.

### Deploy DMARC

DMARC tells receivers what to do when SPF and DKIM fail, and sends you reports.

```dns
_dmarc.example.com.  IN  TXT  "v=DMARC1; p=reject; rua=mailto:dmarc@example.com; ruf=mailto:dmarc-forensic@example.com; adkim=s; aspf=s; pct=100"
```

**Deployment phases:**
1. Start with `p=none` - monitor only, collect reports for 2-4 weeks
2. Move to `p=quarantine; pct=10` - quarantine 10% of failures, increase gradually
3. Graduate to `p=reject` - full enforcement, only after all legitimate mail passes

**Key parameters:**
- `p=` policy: `none` (monitor), `quarantine` (spam folder), `reject` (drop)
- `rua=` aggregate report destination (daily XML reports)
- `adkim=s` strict DKIM alignment (From domain must exactly match DKIM d= domain)
- `aspf=s` strict SPF alignment (From domain must exactly match envelope sender)

> Never jump straight to `p=reject`. The monitoring phase catches legitimate
> senders you forgot about (marketing tools, CRMs, invoicing systems).

### Plan an IP warm-up

From mkurman/zorai, MIT.

More from mkurman/zorai