Guide
11 min read
Email infrastructure is the part of cold outreach that most teams underinvest in and later pay for. The copy is visible and gets iterated on. The infrastructure is invisible until it breaks — at which point months of list-building and campaign investment are wasted on emails that never reached an inbox.
This guide covers the full infrastructure setup: domain selection, DNS configuration, inbox setup, warmup, and ongoing maintenance. Follow it in order. Each step depends on the one before it.
The first principle of cold email infrastructure is to never send cold email from your primary domain. Your primary domain — the one on your website, your business cards, your official communications — carries your brand reputation. If it develops a spam reputation, every email you send from it suffers: customer emails, supplier communications, and marketing alike.
Dedicated sending domains isolate cold email reputation from everything else. If a sending domain gets flagged or its reputation drops, you retire it and add a new one. The primary domain is untouched. This separation is not optional for any serious outbound operation.
Sending domains should look like legitimate business domains, not like they were created to spam people. Good patterns: variations on your company name (acme-mail.com, hireacme.io, acme-outreach.com), domain names that describe what you do (buildingoutbound.io, outbound-ops.com). Avoid: hyphon-heavy names, generic TLDs like .info or .biz, names that look like they were auto-generated.
Purchase two to three domains for every 50-60 campaign sends per day you plan to reach. Each domain will host two inboxes, sending 25-30 emails per day each. Set up all domains at the same time so they can warm in parallel — the warmup period is the same whether you warm one or five.
After purchasing a domain, configure three DNS records before connecting any inboxes: SPF, DKIM, and DMARC. SPF authorises the mail servers that can send email from your domain. Add the SPF record provided by your email provider (Google Workspace or Microsoft 365) to the domain's DNS via your registrar's dashboard.
DKIM adds a cryptographic signature to every email. Your email provider generates a DKIM key pair — add the public key as a TXT record in your DNS. DMARC tells receiving servers what to do when emails from your domain fail SPF or DKIM. Start with a policy of "none" (p=none) to monitor without blocking, then move to "quarantine" and "reject" once you've confirmed everything is passing correctly.
For each sending domain, create two inboxes using Google Workspace or Microsoft 365. Use first name and last name combinations that look like real people: [email protected], [email protected]. The email address should not obviously read as a sending account — first.last@ format is standard and credible.
Configure the inbox display name to match a real person on your team. Enable IMAP access. Connect the inbox to your sending platform (Instantly, Smartlead, or similar) using the app-specific password or OAuth connection your provider supports. Do not start warmup until all inboxes are connected and the sending platform confirms all accounts are active.
Enable warmup for every inbox before sending a single campaign email. Most sending platforms include warmup networks — pools of inboxes that send emails to each other and engage with them, building positive sending history. The platform handles the mechanics; your job is to leave it running for three to four weeks before campaign sends begin.
Start each inbox at ten warmup emails per day. The platform will gradually increase this. Monitor the deliverability score the platform assigns to each inbox — most show a 0-100 score reflecting inbox placement rates. Do not start campaign sending until the score is consistently above 90. A score below 90 means too many of your warmup emails are landing in spam — the inbox is not ready.
Once warmup is complete, set campaign sending limits conservatively: 25-30 emails per inbox per day. This keeps each inbox well inside the threshold that triggers reputation damage. Continue running warmup emails alongside campaign sends — ten warmup emails per day per inbox, running indefinitely.
As your campaign volume grows, add more inboxes and domains rather than increasing per-inbox limits. Inbox rotation — distributing sends across all available inboxes — keeps each one within safe limits while increasing total output. Your sending platform handles this automatically once all inboxes are connected.
Check deliverability metrics weekly: open rate by inbox, bounce rate by domain, spam complaint rate overall. A bounce rate above 5% signals a dirty list — validate emails before they enter sequences. A spam complaint rate above 0.1% signals a relevance problem — the audience is not responding well to the message.
Run a placement test monthly using GlockApps or a similar tool. This sends test emails through your setup and reports how they are categorised — inbox, spam, or promotional — across major email providers. A placement test catches infrastructure problems before they compound into a campaign failure.