National Mattress sells mattresses in the Greater Toronto Area. Northern published the write-up as a deliverability case study. This is an agency page, not a National Mattress post. The numbers below are Northern’s.
They left Mailchimp for Klaviyo after more than two years without a real email program. The list moved. The first campaign opened at 3.2%. Northern says the industry average they were watching sat above 23%. That gap is the story.
If you are looking at the same route, start with the Mailchimp → Klaviyo playbook. Klaviyo’s Mailchimp integration can pull contacts, unsubscribes, cleaned addresses, audiences, and recent open and click data. It does not bring the old sender reputation with it.
Why they moved
Northern says the program had sat still for over two years. The list was full of people who had stopped opening anything. They wanted Klaviyo’s ecommerce automations and segments so they could mail at points in the buying cycle, not blast the whole file.
The page does not name a Mailchimp bill, a list size, or a Klaviyo plan, so I am not inventing those.
The first send told them the copy was not the problem
They migrated, then sent. 3.2% opened. That is a reputation problem, not a subject-line problem.
Northern’s diagnosis is the useful part. Data can move. Inbox placement does not. A domain that has been quiet for two years, then suddenly mails a stale list, looks like a compromised account to the mailbox providers.
How they repaired it
They did not keep blasting the imported file. They built a new sending setup and warmed it.
They put mail on a dedicated sending domain and passed a DMARC policy, so the new reputation sat on infrastructure they controlled. Then they warmed that domain with high-engagement flows and small, targeted sends. They suppressed inactive profiles instead of mailing them. They started a lead-generation campaign to grow a list of people who had just asked to hear from them.
Klaviyo’s current Mailchimp guide is a one-time integration, not a reputation transfer. You connect with an API key. You can sync audiences into Klaviyo lists and pull the last 90 days of campaign engagement on the first historical sync. After that, existing audiences refresh about every 30 minutes. Klaviyo does not import the campaign bodies. Templates get rebuilt. If Shopify still talks to Mailchimp while Klaviyo is also on the store, you can fire double opt-in mail. Disconnect the old store integration first.
Northern does not say whether they used that API sync or a CSV. Mailchimp’s audience export is the other path. Either way, suppressions have to land as suppressions. The playbook treats unsubscribes, complaints, and cleaned addresses as a separate file, never as active rows.
What they gained
Northern’s headlines are a 935% increase in open rate, 391% list growth year over year, and a 33% open rate that they say beat the industry mark they were using.
I am leaving those as agency numbers. I have not seen the raw Klaviyo reports. The 935% lift is the jump off that 3.2% first send, not a claim that every campaign now opens at 33% from day one.
The operational win is clearer than the percents. They could run automations and segments on a warmed domain instead of paying to mail a dead file.
What the agency page skips
Northern does not publish a failed warm-up, a spam-folder screenshot, or a rollback. It also does not say when Mailchimp was turned off, or whether both tools sent for a while.
Campaign history, visual automations, and forms are not in the story. Klaviyo says the integration syncs engagement, not the emails themselves. Journeys get rebuilt. The Mailchimp → Klaviyo playbook already treats automations, billing, and sender reputation as manual work.
The page also does not name which inactives they suppressed, or how many. “Inactive” after a two-year gap could mean most of the file.
If you need to go back, the reverse playbook is Klaviyo → Mailchimp. They did not describe using it.
What I would copy
Do not treat the first imported campaign as a launch. Treat it as a reputation test. If opens collapse, stop and warm.
Export Mailchimp subscribed, unsubscribed, cleaned, and complained contacts as four files. Import only the people who can still be mailed. Suppress the rest in Klaviyo before anyone presses send.
Stand up a dedicated sending domain and wait for DMARC to pass before the first real campaign. Warm with flows that people already asked for, not with a blast.
If Shopify or another store is still wired to Mailchimp, cut that integration before Klaviyo starts collecting the same list.
The Mailchimp → Klaviyo playbook is the checklist I would run. Use Northern’s page for the “list moves, reputation does not” warning, and remember it is an agency case study.