Email Infrastructure for 100K Emails/Month

The 100K/Month Profile

At 100K emails/month, you're beyond hobby-scale but not yet enterprise. This is the volume where:

• Shared IPs still work well
• ESPs offer good value on mid-tier plans
• You need proper authentication but not complex infrastructure
• Deliverability mistakes hurt but are recoverable

Most businesses at this level send a mix of transactional email (20-30%) and marketing email (70-80%). Separating these streams is critical even at this volume.

ESP Selection

Best Options for 100K/Month

ESP	Monthly Cost	Best For	Notes
Postmark	$150	Transactional	Best deliverability, transactional focus
Mailgun	$75	Mixed	Good API, flexible pricing
SendGrid	$60-90	Marketing-heavy	Established, good templates
Amazon SES	$10	Cost-sensitive	More DIY, less support
Brevo	$65	SMB marketing	Built-in CRM features

Decision Framework

Choose Postmark if:

• Most of your email is transactional (receipts, alerts, password resets)
• You'll pay more for better deliverability
• You need excellent support

Choose Mailgun if:

• You need a balance of transactional and marketing
• You prefer API-first integration
• You want straightforward pricing

Choose SendGrid if:

• You're marketing-heavy with templates
• You want a large feature set
• You may need to scale to dedicated IPs later

Choose Amazon SES if:

• You're already in AWS
• You have technical resources to manage it
• Budget is your primary concern

Practitioner note: At 100K/month, I usually recommend Mailgun or SendGrid for most clients. Postmark if transactional dominates. SES only if the client has engineering resources—it's cheap but requires more setup and monitoring than managed ESPs.

Architecture Setup

Basic Structure

yourdomain.com
├── mail.yourdomain.com (transactional)
│   └── ESP: Postmark or dedicated stream
└── marketing.yourdomain.com (marketing)
    └── ESP: SendGrid/Mailgun marketing stream

Even with one ESP, separate the streams:

• Different subdomains
• Different tracking domains
• Different sending pools (if available)

DNS Records

SPF:

mail.yourdomain.com TXT "v=spf1 include:spf.postmarkapp.com ~all"
marketing.yourdomain.com TXT "v=spf1 include:sendgrid.net ~all"

DKIM: Set up per ESP—they provide the records. Usually CNAME to their DNS.

DMARC:

_dmarc.yourdomain.com TXT "v=DMARC1; p=quarantine; rua=mailto:[email protected]"

Start at p=none if you're new to DMARC, advance to quarantine/reject over 4-8 weeks.

IP Strategy

Stay on Shared IPs

At 100K/month (~3,300/day average), you don't have enough volume to:

• Properly warm a dedicated IP
• Maintain consistent daily sending for reputation
• Justify the additional cost

Shared IPs from reputable ESPs have established reputation. Your good sending practices benefit from the pool; the ESP monitors and removes bad senders.

When to Reconsider

Consider dedicated IPs when:

• You consistently send 50K+ emails/day
• Your business depends heavily on email delivery
• You're seeing issues with shared IP reputation (rare with good ESPs)

Sending Best Practices

Volume Distribution

Don't send all 100K emails on one day. Spread consistently:

Pattern	Daily Volume	Weekly Volume	Risk
Steady	3,300/day	23K/week	Low
Weekly blast	25K one day	25K/week	Medium
Monthly blast	100K one day	100K/month	High

Mailbox providers prefer consistent senders. Irregular spikes trigger scrutiny.

List Hygiene

At 100K contacts, you need automated hygiene:

• Hard bounces: Remove immediately
• Soft bounces: Remove after 3 consecutive failures
• Complaints: Remove immediately, investigate patterns
• Inactive: Segment and re-engage or sunset after 6-12 months

Target metrics:

• Bounce rate: <2%
• Complaint rate: <0.1%
• Invalid addresses: <1%

Engagement Segmentation

Send more frequently to engaged users:

Engaged (opened/clicked last 30 days): Weekly emails
Moderate (opened/clicked last 90 days): Bi-weekly
Dormant (no activity 90+ days): Monthly re-engagement, then sunset

Monitoring Setup

ESP Dashboard (Daily when active, weekly otherwise)

Check:

• Delivery rate (should be >95%)
• Bounce rate (should be <2%)
• Complaint rate (should be <0.1%)
• Open rate trends (declining may indicate spam folder)

Google Postmaster Tools (Weekly)

Set up for your domain:

1. Verify domain ownership
2. Check domain reputation
3. Monitor spam rate
4. Review authentication status

At 100K/month, you may not generate enough Gmail traffic for detailed data, but set it up anyway.

Authentication Verification (Monthly)

Use mail-tester.com or similar:

• Send a test email
• Verify SPF passes
• Verify DKIM passes
• Check spam score

Practitioner note: Most deliverability issues at 100K/month come from list quality, not infrastructure. If delivery rates drop, check your list source and hygiene before blaming the ESP. Nine times out of ten, the problem is bad addresses or unengaged recipients.

Cost Breakdown

Example: Mixed Transactional/Marketing

Component	Provider	Monthly Cost
Transactional (30K)	Postmark	$50
Marketing (70K)	Mailgun	$50
Total		$100

Example: Marketing Focus

Component	Provider	Monthly Cost
All email (100K)	SendGrid Essentials	$60-90
Total		$60-90

Example: Budget Option

Component	Provider	Monthly Cost
All email (100K)	Amazon SES	$10
Monitoring tools	Free tier	$0
Total		$10

Budget option requires more manual setup and monitoring.

Common Mistakes at 100K/Month

Buying dedicated IPs too early You don't have the volume to maintain them. Stick with shared.

Sending everything at once Spreading 100K over the month beats blasting it in one day.

Ignoring list hygiene At 100K contacts, a 5% invalid rate means 5,000 bounces per cycle.

Skipping authentication SPF and DKIM aren't optional. DMARC protects your brand.

Not separating transactional One bad marketing campaign shouldn't block password resets.

Scaling Path

When you outgrow 100K/month:

100K → 500K:

• Stay on shared IPs or consider one dedicated IP
• Move to higher ESP tiers
• Implement more sophisticated segmentation

500K → 1M+:

• Consider dedicated IP pool (2-4 IPs)
• Evaluate self-hosted options for cost
• Add dedicated deliverability monitoring

If you're at 100K/month and hitting deliverability issues or planning to scale, schedule a consultation to optimize your setup before problems compound.

Sources

• SendGrid: Pricing and Plans
• Mailgun: Pricing
• Postmark: Pricing
• Amazon SES: Pricing
• Google Postmaster Tools: Getting Started

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v1.0 · March 2026