Cold Email Infrastructure: How to Set Up Domains, Inboxes, DNS and Warmup

Cold email infrastructure is the set of sending domains, inboxes, DNS records, warmup and monitoring that decides whether your emails reach the inbox. The standard setup is a handful of separate domains (never your main one), two to five inboxes per domain, SPF, DKIM and DMARC on every domain, two to four weeks of warmup, low daily volume per inbox and weekly health checks. You can build it yourself, buy done-for-you inboxes from a tool vendor, or hire an agency that owns and runs the whole stack.

What cold email infrastructure includes

Think of it as six layers. If one is weak, the others cannot save you.

Step 1: Buy separate sending domains

Never send cold email from the domain your company uses for everyday mail. If a campaign gets flagged, that reputation hit lands on the same domain that sends your invoices, proposals and support replies.

Instead, register a few domains that are clearly yours. Common patterns:

• Your brand plus a word: getacme.com, tryacme.com, acmehq.com
• Your brand with a different extension, if it fits your market: acme.co
• A short variant: acme-team.com

A few practical rules:

• Stick to mainstream extensions. Instantly's own AirMail service accepts only .com and .org domains, which tells you something about how providers view the rest.
• Forward each domain to your main website. A prospect who types the sending domain into a browser should land somewhere real.

How many domains?

Work backward from volume. Decide how many new prospects you want to contact per day, divide by the number of emails each inbox will send, then divide by inboxes per domain.

Done-for-you providers set their own inbox counts per domain. PlusVibe's Google Workspace offer sets up 3 inboxes and its Microsoft 365 offer 5, and Instantly allows up to 5 accounts per done-for-you domain. Those vendor defaults put most setups at two to five inboxes per domain.

Step 2: Create the inboxes

You have three inbox types to choose from:

• Google Workspace inboxes. The most common choice for cold email. Each inbox is a paid Workspace user.
• Microsoft 365 inboxes. Useful for reaching companies that run on Outlook, and often mixed with Google inboxes.
• SMTP inboxes. Cheaper mailboxes from specialist providers or the tool vendor itself. Instantly's AirMail, for example, runs on Instantly-controlled servers.

Give every inbox a real person's name, a profile photo and a short signature.

Smartlead and PlusVibe can match sender provider to recipient provider (Gmail to Gmail, Outlook to Outlook). If you plan to use that, you need both Google and Microsoft inboxes in the mix.

Step 3: Set up DNS authentication

This is the part most people get wrong, and it is the part mailbox providers check first. Here is what every sending domain needs.

What Google and Microsoft require

Google's sender guidelines apply to everyone who sends to personal Gmail accounts:

• All senders need SPF or DKIM, valid forward and reverse DNS, a TLS connection, and a spam rate below 0.3% in Postmaster Tools. Google recommends staying below 0.1%.
• Bulk senders, meaning more than 5,000 messages a day to Gmail accounts, need SPF, DKIM and DMARC, with the From domain aligned to SPF or DKIM. Marketing and subscribed messages also need one-click unsubscribe.

Microsoft introduced similar rules for senders of 5,000 or more emails a day to outlook.com, hotmail.com and live.com addresses. According to dmarcian's summary, Microsoft moved from sending non-compliant mail to junk to rejecting it outright in May 2025.

Even if you never hit 5,000 a day, set up all three records. It costs nothing.

A quick DNS check before warmup

Before you start warmup, confirm each domain passes this list:

• [ ] One SPF record that includes your inbox provider and your sending tool if required
• [ ] DKIM enabled in Google or Microsoft admin and published in DNS
• [ ] DMARC record live with a reporting address
• [ ] MX records pointing to your inbox provider
• [ ] Custom tracking domain verified in your sending tool (if you track opens or clicks)
• [ ] Domain forwarding to your main site works
• [ ] A test email to a personal Gmail account shows SPF, DKIM and DMARC as "pass" in the message headers

Step 4: Warm up every inbox

New inboxes have no sending history. Warmup builds that history by sending small volumes that get opened and replied to, then slowly raising the number.

What the tools say about timing:

• Instantly recommends a 3-week warmup for its AirMail inboxes. Its standard ramp starts at 2 warmup emails on day one, then 4, then 6, and so on.
• Smartlead says existing accounts need 2+ weeks and new domains can need 1 to 2 months. Its API guide suggests 20 to 40 warmup emails per day, with 40 or fewer for new accounts.
• PlusVibe's default warmup ramp starts at 5 a day, adds 5 a day, and tops out at 50.

Koldify warms inboxes for about three weeks before any campaign sends, and campaigns typically launch around week four. That lines up with the vendor guidance above.

Keep warmup running after campaigns start. Most tools let you share a daily cap between warmup and real sends.

Step 5: Set sending limits and rotation

Volume per inbox is where most setups fail. The fix is simple: more inboxes, fewer emails each.

Reference points from the tools themselves:

Treat the lowest number as your starting point and only raise volume when bounce rates, replies and placement tests say it is safe.

Rotation spreads a campaign across many inboxes so no single inbox carries the load. Instantly, Smartlead and PlusVibe all rotate senders automatically, so you can pause a struggling inbox without stopping the campaign.

Step 6: Monitor and maintain

Infrastructure is not a one-time project. Domains and inboxes wear out, lists decay, and filters change. A weekly routine keeps small problems small.

Weekly checks:

• [ ] Bounce rate per campaign and per inbox (Apollo says a bounce rate of 8% or higher calls for immediate action, so set your own alert threshold lower)
• [ ] Spam rate in Google Postmaster Tools for any domain with enough Gmail volume to report
• [ ] Inbox placement test on a sample of inboxes
• [ ] Blacklist status for each sending domain
• [ ] Warmup health scores in your sending tool
• [ ] Reply rate by inbox (a sudden drop on one inbox usually means it is landing in spam)

Automations worth turning on:

• Smartlead's High Bounce Rate Auto Protection pauses a campaign when bounces hit a threshold you set, such as 3% or 5%.
• PlusVibe can auto-pause sending on high bounce rates, and its placement test add-on can pause sending or reduce warmup when placement drops.
• Instantly's placement automations can pause a mailbox when placement falls below a threshold or it gets blocklisted. It checks 94 blacklists.

Verify every list before it touches an inbox. Bounces are the fastest way to damage a new domain. Apollo refunds the credit when a verified email bounces within 30 days, and PlusVibe and Smartlead include or sell verification credits, so there is no reason to send to unverified addresses.

What it costs: a worked example

Here is a small, realistic setup: 10 sending domains, 3 Google inboxes per domain, 30 inboxes in total. At about 30 cold emails per inbox per business day, that is roughly 20,000 emails a month. Prices are as of October 2026 and come from each vendor's pricing page.

Some tool links here are affiliate links (disclosure).

On top of that you need contact data. Apollo is the usual pick for list building; its Basic plan is $49 per user per month on annual billing and $59 to $65 monthly, depending on the source.

Notice what the table leaves out: the hours. Someone has to set up DNS, watch warmup, build and verify lists, write and test copy, manage replies and replace burned inboxes. That time is the real cost of DIY, and it is covered in more depth in our cold email agency vs DIY breakdown.

Want a second opinion? If you have a setup that is not delivering and you are not sure which layer is broken, book a 30-minute call with David at Koldify and walk through it together.

Three ways to get infrastructure

Option 1: Build it yourself

You buy domains from a registrar, create inboxes in Google Workspace or Microsoft 365, set up every DNS record, then connect everything to a sending tool.

Good fit if: you have someone technical, you want full ownership of every domain and inbox, and you have time to learn from mistakes.

Watch out for: DNS errors that silently fail, SPF records that break the 10-lookup limit, and inbox counts that grow faster than anyone can monitor.

Option 2: Done-for-you inboxes from a tool vendor

The vendor buys the domains, creates inboxes, sets DNS and starts warmup for you. Examples as of October 2026:

Good fit if: you will run campaigns in-house but do not want to touch DNS.

Ask before you buy:

• Can I transfer the domains out? Instantly requires an active Outbound plan to keep DFY accounts and warmup running, and some reviewers complain DFY domains cannot be moved. PlusVibe does not publish transfer terms.
• What happens when I cancel? Per Emelia's Smartlead pricing review, SmartSenders mailboxes keep billing until you delete them, even after you cancel the plan, and domains are non-refundable.
• Who fixes it when placement drops? Done-for-you setup is not done-for-you monitoring.

Our PlusVibe review and Smartlead vs Instantly comparison go deeper on each platform.

Option 3: An agency that owns and runs the infrastructure

A cold email agency handles the infrastructure plus everything that depends on it: lists, copy, sending, monitoring and replies. The important question is whether the agency owns its sending setup or resells someone else's.

Here is how Koldify, the agency behind Nerdy Joe, approaches it:

• Koldify owns and operates its sending infrastructure, with no middlemen.
• It uses separate sending domains, never the client's main domain.
• Inboxes warm for about three weeks before any campaign sends, and campaigns typically launch around week four.
• Lists, offer and copy are built with the client and approved before launch.
• Replies are routed to the client, who handles the conversation, booking and closing.
• It is month-to-month, with no long-term contract.

For a sense of scale, Koldify's own outbound produced about 500 leads from about 250,000 sends in three weeks, and koldify.io reports 10M+ cold emails sent for clients. More detail is on our cold email services page, or you can book a call with David to ask how the setup would work for your market.

Good fit if: you want meetings, not a new technical project, and you have someone ready to handle interested replies.

Not a fit if: you want to own every domain yourself and build in-house skills, you need fewer than a handful of new conversations a month, or you have no one to follow up with interested replies. In those cases, DIY or done-for-you inboxes will serve you better.

Comparing the three options

A simple decision framework

Answer these four questions honestly:

1. Do you have someone who will spend several hours every week on deliverability? If no, rule out pure DIY.
2. Do you have someone who will write, test and rewrite copy and build lists? If no, DFY inboxes alone will not help, because they only solve the setup.
3. Is owning every domain a hard requirement? If yes, choose DIY, or check the transfer terms of any DFY or agency option in writing.
4. Is your goal infrastructure or meetings? If it is meetings and the first two answers were no, an agency is usually the faster path.

Common infrastructure mistakes

• Sending from the main domain "just for a test."
• Starting campaigns on day one instead of after warmup.
• Two SPF records on one domain, which makes SPF fail.
• Pushing volume per inbox instead of adding inboxes.
• Copy that reads like a promotion. Our list of spam trigger words is a good final check before launch.

Compliance basics

Infrastructure is also where compliance lives. In the US, the FTC's CAN-SPAM guidance requires accurate header information, a non-deceptive subject line, a valid physical postal address, a clear way to opt out, and honoring opt-outs within 10 business days. Other countries have stricter rules, so check the law where your prospects are. Instantly, Smartlead and PlusVibe all support a block list or do-not-contact list; use it.

Next step

If you want to run it yourself, start with domains and DNS this week, warm for three weeks, then launch small. If you would rather have a team that owns the infrastructure handle it for you, book a 30-minute call with David and get a plain answer on whether Koldify is the right fit.

Sources

• Gmail Help: Email sender guidelines
• dmarcian: Microsoft enforces SPF, DKIM and DMARC for high-volume senders
• RFC 7208: Sender Policy Framework (SPF)
• FTC: CAN-SPAM Act compliance guide for business
• PlusVibe pricing
• PlusVibe Help: Email warmup
• PlusVibe Help: Done-for-you guide
• PlusVibe Help: Email Recovery Automation
• PlusVibe Help: New features
• G2: PlusVibe pricing
• Instantly pricing
• Instantly Help: Plans overview
• Instantly Help: DFY done-for-you email setup
• Instantly Help: AirMail done-for-you email setup
• Instantly Help: Inbox placement automated tests
• Instantly Help: Custom tracking domain
• Instantly: Email warmup
• Trustpilot: Instantly reviews
• Smartlead pricing
• Smartlead pricing.md
• Smartlead: Email warmup
• Smartlead API guide: Email warmup
• Smartlead Help: Custom tracking domain
• Smartlead Help: High Bounce Rate Auto Protection
• Emelia: Smartlead pricing
• Apollo Knowledge Base: Email sending limits overview
• Apollo Knowledge Base: Data overview
• Apollo Knowledge Base: How Apollo verifies emails
• Valley: Apollo pricing and credits explained (2026)
• Saleshandy: Apollo.io pricing
• Koldify