Most cold email problems get blamed on copy. Then someone checks where the emails landed, and the answer is spam, or nowhere. Infrastructure decides whether anyone sees your message at all, and it is the part teams skip because it is dull and slow to set up.
This post covers the setup we run for clients: how many domains and mailboxes a given volume needs, which DNS records matter, how warmup should go, and the limits we hold. The numbers are practices, not guarantees. Treat them as a starting point and adjust on your own bounce and reply data.
Why your main domain stays out of it
Cold outreach carries risk. Some recipients will mark it as spam no matter how good the email is. If that happens on your primary domain, the damage reaches your invoices, your proposals and your customer replies.
So cold email goes out from secondary domains, close variants of the brand that point to the real website. If a secondary domain gets burned, you retire it and your company email is untouched. We never send cold email from a client's primary domain.
Domains and mailboxes: the volume math
The simple model we use:
- Each secondary domain holds 2 to 3 mailboxes.
- Each warmed mailbox sends 20 to 30 cold emails a day.
- A domain therefore carries roughly 40 to 90 sends a day at the high end of safe.
Work backward from the volume you actually need. A focused program is deliberately smaller than a spray campaign. A hypothetical example: 250 new contacts a week, each getting a four-step sequence over two to three weeks, works out to around 150 to 200 sends a day at steady state. At 25 sends per mailbox that is 6 to 8 mailboxes, which means 3 to 4 domains. Add spare capacity so one flagged mailbox does not stall the campaign.
For comparison, sending 1,000 emails a day takes roughly 12 to 15 domains at the same rates. If a vendor quotes you far fewer domains for that volume, ask what daily limit per mailbox they assume.
The DNS records that matter
Every sending domain needs three authentication records, and they need to be right before the first warmup email goes out.
- SPF lists which servers may send for the domain. One SPF record per domain, never two.
- DKIM signs each message so the receiver can verify it was not altered. Your mailbox provider gives you the key to publish.
- DMARC tells receivers what to do when SPF or DKIM fails and where to send reports. Start with a monitoring policy and tighten later.
Since February 1, 2024, Gmail has required anyone sending more than 5,000 messages a day to Gmail accounts to publish all three records and keep the spam rate reported in Postmaster Tools below 0.10%, never reaching 0.30%. Yahoo began enforcing the same records for bulk senders that month, with a spam rate ceiling of 0.3%. Read the current wording in Google's email sender guidelines and Yahoo's sender best practices, both checked November 9, 2026. Cold senders usually sit under the volume that triggers these bulk rules, but the same filters score everyone, so there is no reason to ship with a weaker setup.
Two more items that get missed:
- A custom tracking domain. Shared tracking links from your sending platform are shared reputation. A branded tracking domain on a CNAME keeps your links yours. Many of our sequences skip click tracking entirely and send plain text.
- A working website on each domain. Redirect the secondary domain to the real site. A bare domain with no content looks like what it is.
Warmup: slow on purpose
A new mailbox has no sending history. Providers watch how it behaves in its first weeks, and a mailbox that goes from zero to 100 emails a day looks like abuse.
Our practice is two to three weeks of warmup before a single prospect email, using the warmup tool in Instantly or Smartlead, then a gradual ramp:
- Weeks 1 and 2 of live sending: 5 to 10 cold emails per mailbox per day.
- Following weeks: add a few sends at a time as bounces stay low and replies come in.
- Cap at 20 to 30 per mailbox per day. We hold that cap even when the platform allows more.
Keep warmup running after launch. It keeps engagement signals healthy while cold volume fluctuates.
What to monitor every week
Infrastructure is not a one-time build. These are the checks we run:
- Bounce rate per campaign. If it passes 3%, pause and re-verify the list. The target is under 2%. Bounces are almost always a list problem, so verify every address before it enters a sequence.
- Inbox placement tests. Send test messages to seed addresses on Gmail and Outlook and see where they land.
- Blacklist checks on each sending domain and its IP, where your provider exposes one.
- Per-mailbox health. If one mailbox drops in placement, rest it instead of pushing more volume through it.
- Spam complaint signals, through Google Postmaster Tools for the domains that send enough to appear there.
When a domain degrades, retire it and bring in a fresh warmed one. Treating domains as consumable is cheaper than trying to rehabilitate a burned one.
What to avoid
- Sending from the primary domain "just for a few tests."
- Buying 30 domains on day one and warming them all identically. Stagger purchase and warmup dates.
- Raising limits to hit a launch date. The date moves, or the domain does.
- Copying a competitor's mailbox count without their list quality, which is what keeps bounces down.
What we do on client builds
Sending infrastructure is part of our Foundation Build, which goes live in 21 days. We buy and set up the secondary domains, publish SPF, DKIM and DMARC and the tracking domain, create and warm the mailboxes, and set rotation and per-mailbox limits in the sending platform. We own that setup, so you never have to touch it, and we watch the whole fleet every week after launch. The build runs while we clean your list and write the first campaign, so sending starts as soon as warmup ends. The full sequence is on how it works, and the programs are on pricing. If you are still deciding between channels, read cold email vs cold calling first.
FAQ
How many domains do I need to start cold email?
It depends on daily volume. Each domain holds two to three mailboxes and each warmed mailbox sends 20 to 30 emails a day, so 150 to 200 sends a day needs three to four domains plus spares. Starting smaller and adding domains as results justify it is safer than buying a big fleet up front.
How long should I warm up a new mailbox?
Two to three weeks of warmup before the first prospect email is our practice, followed by a gradual ramp over the next few weeks. Rushing this is the most common reason new setups land in spam. Keep warmup running after launch so engagement signals stay steady while cold volume changes.
Is it safe to send cold email from my main company domain?
We do not recommend it. A spam complaint wave or a blacklisting on your primary domain affects every email your company sends, including customer and invoice mail. Secondary domains isolate that risk, and a burned one can be replaced without touching your real email.