Email Infrastructure

Email Infrastructure — Deliverability and DNS Authentication

Why Email Authentication Matters

Without SPF + DKIM + DMARC, any server can forge From: noreply@yourdomain.com. Major providers (Google, Microsoft, Yahoo) now reject or spam-folder unauthenticated email. As of 2024, Gmail and Yahoo require DMARC for bulk senders.

MX Records

; Priority lower = higher preference (10 < 20)
example.com.    3600    IN    MX    10    mail1.example.com.
example.com.    3600    IN    MX    20    mail2.example.com.

For relay-only setups (all outbound, no self-hosted inbound), set MX to your relay provider's receiving servers or use a catch-all:

; SendGrid inbound parse MX
example.com.    3600    IN    MX    10    mx.sendgrid.net.

Test MX records:

dig MX example.com
nslookup -type=MX example.com
# From MXToolbox:
# https://mxtoolbox.com/MXLookup.aspx

SPF (Sender Policy Framework)

SPF Mechanisms

SPF Record Examples

; Basic SPF for domain using only SendGrid
example.com.    3600    IN    TXT    "v=spf1 include:sendgrid.net ~all"

; Multiple authorized senders
example.com.    3600    IN    TXT    "v=spf1 ip4:203.0.113.10 include:sendgrid.net include:amazonses.com ~all"

; Include your hosting provider's IP range + relay
example.com.    3600    IN    TXT    "v=spf1 ip4:203.0.113.0/24 include:spf.mailgun.org ~all"

SPF Lookup Limit

SPF allows a maximum of 10 DNS lookups during evaluation. include:, a, mx, ptr, exists each count as a lookup. Exceeding 10 causes permerror, which DMARC treats as a failure.

# Check SPF record and count lookups
dig TXT example.com | grep spf

# Use spf-tools to flatten (replace includes with raw IPs)
# pip install pyspf
python3 -m spf example.com

SPF flattening: replace include:sendgrid.net with the actual IPs from SendGrid's SPF record. Must be re-flattened when SendGrid rotates IPs — use automation or a flattening service.

DKIM (DomainKeys Identified Mail)

DKIM Key Generation

# Generate 2048-bit RSA key pair
openssl genrsa -out dkim_private.key 2048
openssl rsa -in dkim_private.key -pubout -out dkim_public.key

# Extract public key content for DNS TXT record
openssl rsa -in dkim_private.key -pubout -outform der 2>/dev/null | base64 -w0

DKIM DNS TXT Record Format

Selector naming convention: use <year><month> or service name (e.g., sg2024, ses1, mail).

; Full format: <selector>._domainkey.<domain>
sg2024._domainkey.example.com.    3600    IN    TXT    (
    "v=DKIM1; k=rsa; p="
    "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA..."
    "...rest of public key in base64..."
)

Split into multiple strings (DNS TXT records max 255 chars per string):

# Split public key for DNS (many DNS providers do this automatically)
openssl rsa -in dkim_private.key -pubout 2>/dev/null | \
  grep -v "^-" | tr -d '\n' | \
  fold -w 200

DKIM Configuration in Relay Providers

SendGrid: Domains → Add Domain → CNAME records provided (SendGrid manages keys). Custom DKIM requires dedicated IP.

Amazon SES: Email Identities → Verify Domain → DKIM section → Create DKIM (SES manages 2048-bit keys). Three CNAME records provided.

Postmark: Sender Signatures → Add Domain → DKIM TXT records provided.

Test DKIM record:

dig TXT sg2024._domainkey.example.com

# Send a test email and check headers for:
# DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=sg2024; ...
# Authentication-Results: dkim=pass header.d=example.com

DMARC (Domain-based Message Authentication, Reporting & Conformance)

DMARC Record Structure

_dmarc.example.com.    3600    IN    TXT    (
    "v=DMARC1;"
    "p=quarantine;"           ; none | quarantine | reject
    "rua=mailto:dmarc-agg@example.com;"    ; Aggregate reports (daily)
    "ruf=mailto:dmarc-forensic@example.com;"  ; Forensic reports (per-failure)
    "pct=100;"                ; Percentage of messages to apply policy to
    "adkim=s;"                ; DKIM alignment: s=strict, r=relaxed
    "aspf=r;"                 ; SPF alignment: s=strict, r=relaxed
    "sp=reject;"              ; Subdomain policy
    "fo=1"                    ; Forensic options: 0=fail both, 1=fail either, d=DKIM, s=SPF
)

Alignment modes:

• relaxed (default): example.com DMARC aligns with mail.example.com DKIM/SPF — useful for subdomains
• strict: Must be exact domain match

DMARC Rollout Sequence

Step 1: Monitor (week 1–4)

"v=DMARC1; p=none; rua=mailto:dmarc@example.com; pct=100"

Collect aggregate reports. Use dmarcian.com or Postmark DMARC Digests to parse reports. Identify all legitimate senders failing DMARC.

Step 2: Fix Sending Sources

• Ensure all authorized mail goes through DKIM-signed relay
• Add any missed senders to SPF
• Remove unknown/unauthorized sending sources

Step 3: Quarantine (week 4–8)

"v=DMARC1; p=quarantine; rua=mailto:dmarc@example.com; pct=10"

Start at 10% — only 10% of failing mail goes to spam. Increase pct by 10% each week while monitoring reports.

Step 4: Full Quarantine

"v=DMARC1; p=quarantine; rua=mailto:dmarc@example.com; pct=100"

Step 5: Reject (week 8–12)

"v=DMARC1; p=reject; rua=mailto:dmarc@example.com; pct=10"

Again, ramp pct up weekly.

Step 6: Full Enforcement (target)

"v=DMARC1; p=reject; rua=mailto:dmarc@example.com; ruf=mailto:dmarc-forensic@example.com; pct=100; adkim=s; aspf=r"

PTR (Reverse DNS)

PTR is a reverse DNS record: <reversed-IP>.in-addr.arpa. → hostname.

Mail servers check PTR of the sending IP and compare to the From: or EHLO hostname. PTR mismatch is a strong spam signal.

# Check PTR for an IP
dig -x 203.0.113.10
nslookup 203.0.113.10

# Check what your IP's PTR resolves to
curl ifconfig.me   # Get your outbound IP
dig -x $(curl -s ifconfig.me)

To set PTR: contact your hosting provider or VPS control panel (e.g., DigitalOcean: Droplet → Settings → rDNS; Hetzner: Server → Networking → IP settings). PTR must match the EHLO hostname your mail server uses.

BIMI (Brand Indicators for Message Identification)

Requires: DMARC p=reject + a Verified Mark Certificate (VMC from DigiCert/Entrust).

; BIMI record (hosted SVG logo + VMC certificate URL)
default._bimi.example.com.    3600    IN    TXT    (
    "v=BIMI1;"
    "l=https://cdn.example.com/logo.svg;"
    "a=https://cdn.example.com/vmc.pem"
)

SVG requirements: SVG Tiny PS format, square, brand-safe, publicly accessible HTTPS URL.

Complete DNS Record Set: SendGrid Integration

; ── MX Records (using SendGrid inbound parse) ──────────────────────────────────
example.com.              3600    IN    MX     10    mx.sendgrid.net.

; ── SPF Record ──────────────────────────────────────────────────────────────────
example.com.              3600    IN    TXT    "v=spf1 include:sendgrid.net ~all"

; ── DKIM (SendGrid provides these CNAME records, not TXT) ────────────────────────
; SendGrid uses CNAME delegation to manage their own DKIM signing
em1234.example.com.       3600    IN    CNAME  u1234.wl.sendgrid.net.
s1._domainkey.example.com. 3600   IN    CNAME  s1.domainkey.u1234.wl.sendgrid.net.
s2._domainkey.example.com. 3600   IN    CNAME  s2.domainkey.u1234.wl.sendgrid.net.

; ── DMARC Record (start with p=none monitoring) ───────────────────────────────────
_dmarc.example.com.       3600    IN    TXT    "v=DMARC1; p=none; rua=mailto:dmarc@example.com; pct=100"

; ── PTR (set via hosting provider control panel) ─────────────────────────────────
; 10.113.0.203.in-addr.arpa. → mail.example.com

SMTP Relay Services

Amazon SES

# SMTP credentials (different from IAM credentials — generate in SES console)
# SES SMTP endpoint: email-smtp.us-east-1.amazonaws.com:587 (TLS)

# Verify domain in SES (generates DKIM CNAME records)
aws ses verify-domain-identity --domain example.com
aws ses get-identity-dkim-attributes --identities example.com

# Move out of sandbox (required for sending to unverified addresses)
# Request via SES console: Account dashboard → Request production access

# Configure bounce + complaint handling (critical for reputation)
aws ses set-identity-notification-topic \
  --identity example.com \
  --notification-type Bounce \
  --sns-topic arn:aws:sns:us-east-1:123456789:ses-bounces

aws ses set-identity-feedback-forwarding-enabled \
  --identity example.com \
  --forwarding-enabled    # Redirect to verified email if no SNS topic

SendGrid

# API key (Settings → API Keys → Create)
# Scopes: Mail Send (minimum), plus Mail Settings for bounce management

# Test send via curl
curl -X POST https://api.sendgrid.com/v3/mail/send \
  -H "Authorization: Bearer $SENDGRID_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "personalizations": [{"to": [{"email": "test@example.com"}]}],
    "from": {"email": "noreply@example.com"},
    "subject": "Test",
    "content": [{"type": "text/plain", "value": "Test email"}]
  }'

Postmark

Separate streams for transactional (receipts, passwords) and broadcast (newsletters). Transactional messages get priority delivery queues.

# SMTP: smtp.postmarkapp.com:587 (STARTTLS)
# API token: Server Settings → API Tokens
# From address must be verified sender signature

Postfix Basics (Smart Relay Host)

/etc/postfix/main.cf (key relay settings):

myhostname = mail.example.com
mydomain = example.com
myorigin = $mydomain
inet_interfaces = loopback-only    # Only accept local submissions
mydestination =                    # Not a final destination — relay only

# Relay all mail through SendGrid SMTP
relayhost = [smtp.sendgrid.net]:587

# SASL authentication for relay
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

# Limits
smtp_destination_concurrency_limit = 5
smtp_destination_rate_delay = 1s

/etc/postfix/sasl_passwd:

[smtp.sendgrid.net]:587    apikey:YOUR_SENDGRID_API_KEY

sudo postmap /etc/postfix/sasl_passwd
sudo chmod 600 /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
sudo systemctl restart postfix

# Test send
echo "Test body" | mail -s "Test subject" test@example.com

# Check mail queue
mailq

# Force retry stuck queue
sudo postqueue -f

# Remove all queued mail (careful!)
sudo postsuper -d ALL

Deliverability Testing Tools

# Verify authentication results from a received email header:
# DKIM verification
dig TXT s1._domainkey.example.com

# Check SPF evaluation for a specific IP
dig TXT example.com | grep spf
# Use: https://www.kitterman.com/spf/validate.html

# Check DMARC policy
dig TXT _dmarc.example.com

Anti-Patterns

Troubleshooting