Cold email infrastructure is the technical foundation—domain portfolio, DNS authentication records, warmed mailboxes, and sending platform configuration—that determines whether your outbound emails land in the primary inbox or the spam folder at scale. Get it right and you have a repeatable pipeline engine. Get it wrong and ISPs blacklist your domains within weeks, killing deliverability across your entire sending operation.
Why Most Outbound Teams Break at Scale
The average SDR-led outbound program runs 20–30 emails per day per rep from a single company domain. That model works at low volume. At 100K sends per month it fails predictably: your company root domain gets flagged, inbound email delivery degrades alongside outbound, and your ops team spends weeks cleaning up a mess that was entirely preventable.
The answer is a dedicated sending infrastructure built on purpose-created domains, properly authenticated, warmed systematically, and distributed across sending IP pools that are isolated from your brand's primary communication stack.
According to Validity's 2025 Email Deliverability Benchmark Report, campaigns with full SPF, DKIM, and DMARC alignment see inbox placement rates 23 percentage points higher than those without—yet fewer than 40% of B2B outbound senders have all three records correctly configured.
Domain Procurement Strategy
How Many Domains Do You Need?
The math is straightforward. Each properly warmed mailbox should send no more than 30–40 cold emails per day to maintain healthy deliverability. Most sending platforms support 2–3 mailboxes per domain before IP-level signals start clustering and raising flags.
Work backward from your target volume:
- 100K sends/month = ~3,333 sends/day
- At 35 sends/inbox/day = 95 inboxes minimum
- At 2 inboxes/domain = 48–50 domains minimum
- Add 20% buffer for rotation and rest days = 58–60 domains
For a program targeting 100K monthly sends, budget for 60 active sending domains with 2 mailboxes each. This gives you operational headroom to rotate domains in and out as reputation signals fluctuate, retire domains approaching 12 months of active use, and expand volume without a standing-up bottleneck.
Domain Naming Conventions
Sending domains must be clearly distinct from your primary company domain but plausibly related to your brand or use case. Avoid anything that looks machine-generated or keyword-stuffed—ISPs and spam filters are trained to pattern-match on exactly that.
Proven naming patterns for a company called "Acme Corp" (acmecorp.com):
| Pattern | Example | Risk Level |
|---|---|---|
| Brand + modifier | acme-hq.com, acme-labs.com | Low |
| Brand + region/team | acme-us.com, acme-sales.com | Low |
| Product/vertical | acmegrowth.com, acmerevops.com | Medium |
| Unrelated coined domain | getacme.io, tryacme.co | Medium |
| Generic keyword | b2bsales.com, outreachteam.com | High |
Register domains across multiple TLDs (.com, .io, .co, .net) to diversify your pool. Avoid .biz, .info, or newer gTLDs—they carry higher spam association baseline rates. Purchase from registrars that support easy DNS management: Namecheap, Cloudflare Registrar, or Google Domains work well operationally.
Domain Age and Reputation
Freshly registered domains carry zero reputation, which cuts both ways. ISPs default to treating new domains with suspicion, which is why warming exists. However, you can sometimes purchase aged domains (2–5 years old, previously used for legitimate purposes) through domain brokers or auction platforms like GoDaddy Auctions. An aged domain with clean history can shorten your warming timeline by 2–3 weeks.
Before purchasing any aged domain, check it against MXToolbox, Google Postmaster Tools (register it first), and Barracuda's reputation lookup. A domain with prior spam history is worse than a fresh registration.
DNS Configuration: SPF, DKIM, and DMARC
Every sending domain needs three DNS records configured correctly before a single warm-up email leaves the inbox. This is non-negotiable and takes less than 30 minutes per domain if you have a documented runbook.
SPF (Sender Policy Framework)
SPF authorizes which mail servers can send email on behalf of your domain. The record lives as a TXT record at your root domain.
Standard SPF for Instantly/Smartlead with Google Workspace:
v=spf1 include:_spf.google.com include:spf.protection.outlook.com ~all
Key rules:
- Keep lookups under 10 DNS lookups (SPF lookup limit is hard-set at 10)
- Use
~all(softfail) during initial setup; move to-all(hardfail) after 30 days of confirmed legitimate sending - If using multiple sending platforms, each
include:costs one lookup
DKIM (DomainKeys Identified Mail)
DKIM adds a cryptographic signature to outgoing emails, allowing receiving servers to verify the email wasn't tampered with in transit. Most email sending platforms (Google Workspace, Microsoft 365, Instantly, Smartlead) generate DKIM keys for you—you publish the public key as a CNAME or TXT record.
For Google Workspace sending domains:
- Admin Console > Apps > Google Workspace > Gmail > Authenticate Email
- Generate a new 2048-bit DKIM key for each domain
- Add the generated TXT record to DNS (propagation: 24–48 hours)
- Start signing after propagation confirms
Use 2048-bit keys, not 1024-bit. Microsoft and Google both deprecated 1024-bit DKIM support in 2024, and several major ISPs now treat 1024-bit DKIM as an authentication failure.
DMARC (Domain-based Message Authentication, Reporting and Conformance)
DMARC tells receiving servers what to do when SPF or DKIM checks fail, and instructs them to send reports back to you. Start with a monitoring-only policy and tighten over 60 days.
Week 1–4 (monitoring):
v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com; ruf=mailto:dmarc@yourdomain.com; fo=1
Month 2 (quarantine):
v=DMARC1; p=quarantine; pct=25; rua=mailto:dmarc@yourdomain.com
Month 3+ (enforce):
v=DMARC1; p=reject; rua=mailto:dmarc@yourdomain.com
Use a DMARC reporting tool—Dmarcian, Valimail, or EasyDMARC—to parse aggregate reports. At 60 domains you will receive hundreds of XML reports monthly; parsing them manually is not feasible.
Inbox Warming: Timelines and Tools
Warming is the process of establishing sender reputation by starting with low volumes of email that generate positive engagement signals (opens, replies, not-spam marks) and gradually increasing volume over time. No warming = no reputation = no inbox placement.
Warming Timeline by Platform
Google Workspace (recommended for highest inbox rates):
| Week | Daily Send Limit | Notes |
|---|---|---|
| 1–2 | 5–10/day | Warm-up tool only, no cold outreach |
| 3–4 | 15–25/day | Warm-up tool + light cold outreach (5–10 cold/day) |
| 5–6 | 30–40/day | Full cold outreach, monitor open rates |
| 7+ | 35–40/day | Steady state; never exceed 50/day on Google |
Microsoft 365 (Outlook):
Microsoft limits are tighter than Google for new domains. Expect a 6–8 week warm-up before hitting 35 sends/day without Outlook-specific deliverability degradation. The tradeoff is that Outlook-hosted sending often achieves higher inbox rates at enterprise accounts, making it worth the longer ramp.
Warming Tools
Three platforms dominate the market:
Instantly Warmup — Built into the Instantly sending platform. Uses a network of real inboxes to simulate sending activity. Warmup emails automatically land in inbox, get opened, and get marked as not-spam. Works well if you are already on Instantly for sending.
Mailwarm — Standalone warming tool with a 35,000+ inbox warming network. Pricing is per mailbox per month. Good choice if you are sending via Smartlead or another platform that doesn't have native warming.
Lemwarm (by Lemlist) — Similar network-based warming with built-in deliverability scoring. Integrates natively with Lemlist but works as a standalone product. Includes a deliverability dashboard that tracks domain reputation signals from Google Postmaster.
Run warming on every mailbox for the full duration of the warm-up period—do not stop warming when you hit full send volume. Continuous low-level warming activity (5–10 warm emails/day) maintains reputation signals on active sending domains.
Warming Benchmarks
A properly warmed inbox on Google Workspace should hit the following before you move to full cold outreach volume:
- Open rate on warm-up emails: 80%+
- Spam rate on warm-up emails: <0.1%
- Domain reputation in Google Postmaster: High (not Medium, not Low)
- Deliverability score in warming platform: 85/100+
If any metric is below threshold at week 6, extend warming by 2 weeks before scaling cold sends. Rushing this is the number one cause of domain burnout.
Sending Volume Ramp-Up Schedule
Even after warming, you must ramp cold outreach sends gradually. Moving from 0 to 35 cold emails/day overnight on a freshly warmed domain is a strong spam signal.
8-Week Cold Ramp Schedule (Per Mailbox)
| Week | Cold Sends/Day | Warm Sends/Day | Total/Day |
|---|---|---|---|
| 1 | 5 | 10 | 15 |
| 2 | 10 | 10 | 20 |
| 3 | 15 | 8 | 23 |
| 4 | 20 | 8 | 28 |
| 5 | 25 | 7 | 32 |
| 6 | 30 | 7 | 37 |
| 7 | 35 | 5 | 40 |
| 8+ | 35–40 | 5 | 40–45 |
Scale your domain pool to match ramp schedule. If you need 95 mailboxes at full volume, stand up domains in phases:
- Month 1: Stand up 20 domains (40 mailboxes), begin warming
- Month 2: Stand up 20 more domains, begin warming; Month 1 domains enter full cold ramp
- Month 3: Stand up remaining 20 domains; full portfolio operational at scale
This phased approach means you are consistently at some level of warm-up or ramp while never going from zero to full volume on any domain.
B2B Outbound Systems
End-to-end outbound infrastructure built for mid-market scale—domain setup, enrichment, sequences, and deliverability monitoring included. See how it works
Data Enrichment Services
Waterfall enrichment across Apollo, ZoomInfo, Clay, and Clearbit to maximize contact data coverage before a single email sends. Explore enrichment
Deliverability Checker
Audit your DNS records, check domain blacklist status, and get a sender reputation score in under 60 seconds. Run a free check
Mailbox Provider Rotation
At 100K sends per month, sending from a single provider (e.g., all Google Workspace) creates concentration risk. A single policy change at Google—or a reputation event that triggers manual review—can take down your entire sending operation simultaneously.
Distribute your mailbox portfolio across at least two providers:
Recommended Split (100 mailboxes):
- 60 mailboxes: Google Workspace
- 30 mailboxes: Microsoft 365 (Outlook)
- 10 mailboxes: Private SMTP (Mailgun, SendGrid, or self-hosted)
Why this split? Google Workspace achieves the highest inbox placement rates for SMB and mid-market targets. Microsoft 365 performs better at enterprise accounts (Fortune 5000) where Outlook is the dominant mail client. Private SMTP fills in for high-volume batch sends (newsletters, re-engagement) where inbox placement is less critical than throughput.
Segment your prospect list by the likely email infrastructure of the target company. Large enterprises predominantly use Microsoft 365—send to those accounts from Microsoft-hosted mailboxes. Mid-market and SMB skews heavily toward Google Workspace.
Tools like Hunter.io's email verifier and Apollo's enrichment layer will often surface the MX provider for a contact's domain, allowing you to route sends by provider at the sequence level.
Monitoring and Reputation Management
At 100K monthly sends, something will go wrong every month. The question is whether you catch it in 24 hours or after 30 days of degraded deliverability.
Core Monitoring Stack
Google Postmaster Tools — Free, essential. Set up every sending domain in Postmaster. Monitor domain reputation (High/Medium/Low/Bad), spam rate, and IP reputation daily. Reputation dropping from High to Medium is your early warning signal. Do not wait for Low or Bad to take action.
MXToolbox Blacklist Monitor — Checks your sending domains and IPs against 100+ spam blacklists on a scheduled basis. Set alerts for any blacklist hit. A single Spamhaus listing can tank deliverability by 40–60% immediately.
Smartlead or Instantly Analytics — Both platforms surface per-mailbox open rates, reply rates, and bounce rates. Create alerts for any mailbox with a bounce rate above 3% or an open rate below 10%—both are symptoms of reputation issues or list quality problems.
EmailListVerify or NeverBounce — Verify every prospect list before it enters a sequence. A list with more than 2% invalid addresses will generate hard bounces that damage sender reputation faster than almost anything else. Budget $50–100/month for list verification at this volume.
Reputation Recovery Protocol
If a domain hits a blacklist or drops to Medium reputation in Google Postmaster:
- Immediately pause cold sends on affected domains (within 24 hours of detection)
- Ramp up warming to 15–20 warm emails/day to rebuild positive engagement signals
- Review recent sends for list quality issues, unusually high bounce or complaint rates
- Submit delisting requests to any specific blacklists (Spamhaus, Barracuda, SURBL)
- Hold 2–3 weeks before resuming cold sends at reduced volume (50% of previous max)
- Retire the domain if reputation does not recover to High within 45 days
Domain retirement is not failure—it is planned infrastructure management. A domain with 12 months of active cold sending has a statistically elevated risk of degraded reputation. Budget for domain rotation as an operational expense, not an emergency response.
Deliverability KPIs to Track Weekly
| Metric | Healthy Threshold | Action Required |
|---|---|---|
| Inbox placement rate | >85% | Audit DNS, review list quality |
| Open rate (cold) | 35–55% | Below 25% = deliverability issue |
| Bounce rate | <2% | Above 3% = pause and verify list |
| Spam complaint rate | <0.08% | Above 0.1% = ISP compliance risk |
| Google Postmaster reputation | High | Medium = reduce volume; Low = pause |
| Active blacklist hits | 0 | Any hit = immediate pause |
For a complete approach to diagnosing and resolving deliverability issues once your infrastructure is running, see our deep-dive: Cold Email Deliverability Troubleshooting Guide.
Tooling Stack Summary
Here is the tooling stack we recommend at the 100K sends/month tier, based on production systems we have built for SEP and similar mid-market outbound programs:
Sending Platforms:
- Instantly — best for high-volume cold outreach with built-in warming and inbox rotation
- Smartlead — strong multi-mailbox management and analytics; better for teams managing multiple client accounts
List Building and Enrichment:
- Apollo.io — primary prospecting and contact enrichment
- Clay — waterfall enrichment and AI-personalization at scale
- ZoomInfo — firmographic depth and intent data for enterprise ICP
Deliverability and Monitoring:
- Google Postmaster Tools — domain reputation monitoring (free)
- Mailwarm or Instantly Warmup — continuous inbox warming
- MXToolbox Monitor — blacklist and DNS health monitoring
- Dmarcian — DMARC report parsing
Verification:
- NeverBounce or EmailListVerify — pre-send list validation
For domain warming strategy specifically—including how to sequence warm-up email content and manage provider-level reputation signals—see our companion guide: Domain Warming Strategy for 2026.
If you want to calculate the ROI of a properly built outbound infrastructure against your current cost-per-opportunity, use our ROI Calculator to model the impact before you commit.
Case Study: 100K Sends at OppZo
OppZo needed to reach a highly specific ICP across a fragmented market—small business owners eligible for government contract financing. The challenge was not writing copy; it was building infrastructure capable of reaching 80,000+ decision-makers monthly without triggering ISP-level blocks at scale.
We stood up a 55-domain portfolio across Google Workspace and Microsoft 365, implemented a waterfall enrichment workflow in Clay to source and validate contact data, and ran a 6-week phased warm-up before scaling sends. The result was a sustained 42% average open rate at 80K monthly sends—with a Google Postmaster domain reputation that held at High throughout the engagement.
The infrastructure architecture from that program is the basis for the framework in this guide.
FAQ: Cold Email Infrastructure
How many domains do I need to send 100,000 cold emails per month?
Plan for 55–65 sending domains with 2 mailboxes each. At a safe limit of 35 cold emails per inbox per day and 22 sending days per month, 60 domains with 2 mailboxes each gives you approximately 92,400 sends per month with buffer for rest days and domain rotation. Add a 10–15% domain buffer for domains you are actively warming, resting, or retiring.
What is the minimum warm-up time before sending cold emails?
Four weeks is the absolute minimum for Google Workspace domains before beginning cold outreach—and even then you should start at no more than 10–15 cold sends per day. A full 6-week warm-up with a gradual cold ramp to 35/day is the standard we use in production. Rushing warm-up is the primary cause of domain burnout and deliverability failure in the first 90 days.
Should I use Google Workspace or Microsoft 365 for cold email sending?
Use both. Google Workspace achieves higher inbox placement rates for SMB and mid-market targets (roughly 60–65% of the B2B market). Microsoft 365 outperforms at large enterprise targets where Outlook dominates. A split of roughly 60% Google / 30% Microsoft / 10% private SMTP provides redundancy and optimizes placement by target company size. Never concentrate 100% of sends on a single provider.
What DNS records are required for cold email sending domains?
All three: SPF, DKIM, and DMARC. SPF authorizes your sending servers; DKIM provides cryptographic signing that proves email authenticity; DMARC sets enforcement policy and routes reports back to you. Domains missing any of these records see 20–40% lower inbox placement in controlled tests. Additionally, configure MX records even on sending-only domains—domains without MX records are flagged as suspicious by many ISPs. Run your DNS configuration through our Deliverability Checker to validate all records before warming.
When should I retire a sending domain?
Retire a domain when: (1) it hits Low or Bad reputation in Google Postmaster and does not recover to High within 45 days of reduced sending; (2) it appears on a major blacklist (Spamhaus SBL, Barracuda) and the delisting request fails or the domain relists within 30 days; or (3) it has been in active cold sending for 12+ months. Domain retirement is planned infrastructure management, not failure—budget for it as an operational cost and maintain a 20% domain buffer to replace retired domains without volume interruption.
Ready to Build Infrastructure That Actually Scales?
Most outbound programs fail at the infrastructure layer, not the copy layer. If you are running more than 20K monthly sends and have not formally architected your domain portfolio, DNS setup, and warming cadence, you are running on borrowed time.
Hyperspect.AI has built production outbound infrastructure for 500+ B2B companies across the $20M–$200M ARR range. We handle domain procurement, DNS configuration, warming, sending platform setup, and ongoing reputation monitoring as part of our B2B Outbound Systems engagement.
We also build the data layer—waterfall enrichment, ICP list construction, and contact validation—through our Data Enrichment service, so the list that enters your infrastructure is clean before the first send.
Schedule a systems architecture call and we will audit your current infrastructure and show you exactly what needs to change to hit 100K sends per month without burning your domain portfolio.