If your brand is doing $300,000 or more a month and you already run Klaviyo, you don’t need someone to introduce you to email marketing. You need a partner who can look at your account, tell you what’s actually broken, and fix the parts of your retention system that are leaving revenue on the table. That’s a different hiring decision than picking “an email agency,” and most shortlists online don’t treat it that way.
This list is built for that decision. We’re not ranking agencies by who has the flashiest homepage or the most case studies with vague percentages attached. We’re using criteria you can actually verify: confirmed Klaviyo partner status, case studies with real business metrics in a comparable context, the metrics an agency holds itself accountable to, and whether they think about retention as a system or just a channel.
What we’ll cover
- How Klaviyo’s partner ecosystem actually works, and why a badge alone isn’t a verdict
- A full breakdown of Retention Side, including documented results and where it fits
- Credible alternatives for different revenue ranges, models, and regions
- A decision framework for choosing between them
- What any Klaviyo agency should be accountable for, beyond open rates
- Answers to the most common questions about Klaviyo agencies and the wider agency landscape
How Klaviyo’s partner ecosystem actually works
Klaviyo doesn’t operate a small curated list of approved agencies. Its partner program includes more than 6,000 agency partners globally, serving a platform with over 193,000 high-intent brands. Partners are ranked into tiers, Silver, Gold, Platinum, Platinum Master, Elite, and Elite Master, based largely on managed revenue and client volume running through the platform.
The official partner directory is the right starting point for verification. It’s public, it’s searchable, and it lists agencies by current tier. That matters because tier badges get pasted into decks and homepages long after they’ve expired or changed. Before you take any agency’s claimed status at face value, check it against the live directory.
Here’s the part worth understanding as an operator: a high tier tells you an agency manages meaningful Klaviyo spend and revenue. It does not tell you whether that agency understands your category, whether their process fits how your customers behave, or whether they measure success the way you need to. Tier is a floor for credibility, not a substitute for due diligence. Treat the directory as step one of vetting, not the final answer.
Retention Side: a system-level retention partner for established brands
Retention Side is a Klaviyo-focused ecommerce retention agency built for brands that have outgrown “send more campaigns” as a strategy. The starting point is Klaviyo email, but the actual scope is broader: the agency operates as a strategic and implementation partner across the full retention system, expanding into SMS, push notifications, direct mail, loyalty programs, WhatsApp, and Viber based on what an audience actually responds to.
The work is organized around four pillars: deliverability, list growth, automated flows, and campaign strategy. Deliverability comes first because none of the rest matters if email isn’t reaching the inbox. From there, the process follows five stages: foundational research, optimization workshops, retention strategy design, a controlled A/B testing framework, and ongoing behavioral data analysis. This isn’t a one-time setup and handoff model. Flows and campaigns get revisited and improved on an ongoing basis, because customer behavior and inbox conditions both shift over time.
Retention Side works across a range of consumer categories, including beauty and skincare, apparel and footwear, supplements and nutrition, pet care, sports and outdoors, jewelry and luxury, home and kitchen, baby and kids, electronics, and groceries and snacks. Documented results from its own case studies include email-attributed revenue increasing 800% within the first 60 days without any increase in campaign volume, an additional $250,000 in revenue within 120 days after fixing deliverability issues and rebuilding flows, a returning customer rate that moved from 21.4% to 29.8% over six months, a 35% reactivation rate on a previously dormant list, and an 8% reduction in return order rate driven by clearer sizing communication in email. These are first-party results, not independently audited, and any agency’s published case studies should be read with that caveat. But they reflect the specific outcomes the agency holds itself accountable to: returning customer rate, retention-channel revenue, deliverability health, and list quality, not open and click rates in isolation.
For a brand doing $300K+/month that wants ownership of the entire retention system rather than a vendor who executes send requests, this is the profile to evaluate first. The honest limitation: if you strictly want a low-cost, campaign-only execution shop and have no interest in cross-channel expansion, that’s a narrower need than what Retention Side is built to solve.
Credible alternatives for different needs
No single agency fits every brand, and a fair shortlist should say so directly. The following are drawn from Klaviyo’s own partner directory and, where noted, a third-party industry roundup that has not been independently verified. Confirm current tier status on connect.klaviyo.com before making a final decision, since tiers change.
Domaine holds Klaviyo Elite Master status and runs lifecycle and CRM programs across email, SMS, push, and WhatsApp for a roster that includes Milk Makeup, Alpinestars, Daily Harvest, Laura Mercier, The Row, Dollar Shave Club, Karl Lagerfeld, The Body Shop, Olaplex, and Marc Fisher. Their public positioning centers on measurable impact within the first 90 days. This is a strong fit for larger, multi-brand, or omnichannel retailers that need enterprise-grade CRM infrastructure alongside lifecycle execution.
Retention Harbor is a Klaviyo Platinum Master agency built specifically around retention systems, using a proprietary process it calls “Revenue Anchor.” It works with more than 20 ecommerce brands, several generating over $50 million a year, and offers both full-program management and system rebuilds. Brands at the higher end of the revenue spectrum that want a partial-involvement or rebuild-focused engagement should look here.
Threadpoint, a Klaviyo Platinum Master, positions itself against templated campaigns and discount-dependent strategies, drawing on a team with 20+ years in performance marketing. Absolute Web, an Elite Master partner operating since 1999 with offices in Miami, LA, and Lisbon and multiple Inc. 5000 appearances, treats email and SMS automation as one service line inside a broader ecommerce build, CRO, and growth offer. Both suit brands that want a full-service agency relationship rather than a retention specialist.
Flowium is described by a 2026 third-party roundup as the largest pure-play email agency at the Elite tier, a fit for brands that want high-volume email execution without a broader system mandate. That same roundup, published by YOCTO, an agency evaluating itself alongside competitors, lists Underground Ecom (claimed 2025 Klaviyo Agency Partner of the Year for EMEA), Get Better (UK), Mailody (DACH region), and EmailClub (France) as regional Elite-tier options. Treat this list as a lead worth investigating, not independent verification, since self-published rankings carry an obvious bias toward the publisher. Verify each agency’s current tier directly before shortlisting them.
One name worth flagging honestly: Magnet Monster is a known specialist in the retention and Klaviyo space, but we did not collect verified first-party evidence, case studies, or current tier status for them in this research cycle. If they’re on your list, vet them with the same criteria you’d apply to anyone else here.
How to choose between them
Start with revenue range and complexity. A single-brand company doing $300K to $2M a month has different needs than a multi-brand portfolio doing $10M+, and agencies calibrate their processes around the client size they serve most often. Asking an enterprise-focused agency to run a lean, single-brand account (or vice versa) usually produces a mismatch in attention and pricing.
Next, match business model to agency specialty. Subscription and replenishment brands need different flow logic and churn-prevention thinking than one-time-purchase brands built on new-customer acquisition and occasional repeat orders. An agency’s case studies should reflect your model, not just your category.
Consider region if compliance, timezone overlap, or local market nuance matters to how you operate, particularly for SMS regulations and customer communication norms outside the US. And be honest about your internal team’s maturity. If you have a strong in-house lifecycle marketer who just needs execution support, you want a different engagement than if you need someone to own strategy end to end.
The single most useful screening question: ask what metrics they report on by default. An agency that leads with open rate and click rate in a sales call is measuring vanity engagement. An agency that leads with returning customer rate, retention-channel revenue contribution, deliverability health, and list growth quality is measuring the things that actually move your P&L.
What a Klaviyo agency should be accountable for
Open rates and click rates are diagnostic, not outcomes. They tell you whether a subject line worked, not whether your retention system is healthy. A serious Klaviyo agency should be reporting against a different scoreboard.
Returning customer rate is the clearest signal that lifecycle work is functioning, because it reflects whether customers are actually coming back to buy again. Retention-channel revenue growth, the share of revenue coming from email, SMS, and other owned channels over time, shows whether the system is becoming a larger and more reliable part of the business, not just a side channel. Deliverability health, meaning inbox placement, spam complaint rate, and bounce rate, determines whether messages are even reaching customers in the first place; without it, every other metric is unreliable. And list growth needs to be evaluated on lead-to-customer conversion quality, not raw subscriber count, since a large low-intent list produces worse economics than a smaller, well-matched one.
If an agency can’t speak fluently to these four areas in a sales conversation, that’s a meaningful signal about how they’ll operate once you’re a client.
FAQ
What are the top Klaviyo agencies?
There’s no single authoritative ranking, because “top” depends on the metric you use, revenue managed, client roster, geographic focus, or specialization. The most defensible approach is to start from Klaviyo’s own partner directory at connect.klaviyo.com and filter by tier, then evaluate the agencies that surface against business-metric case studies in your category. Based on verified partner status and case study depth, agencies worth evaluating in 2026 include Retention Side for brands wanting full retention-system ownership, Domaine for enterprise multi-brand lifecycle work, Retention Harbor for large-brand retention rebuilds, and Threadpoint, Absolute Web, and Flowium for full-service or high-volume email execution. Regional Elite-tier options like Underground Ecom, Get Better, Mailody, and EmailClub are worth investigating but should be verified independently before you commit.
What are the big 5 marketing agencies?
This usually refers to the five global advertising and marketing holding companies, not Klaviyo or retention specialists: WPP, Omnicom, Publicis Groupe, Interpublic Group (IPG), and Dentsu. Based on FY2024 primary financial releases, WPP reported revenue of £14,741 million, Omnicom reported revenue of $15.7 billion with 5.2% organic growth, Interpublic reported total revenue including billable expenses of $10.7 billion, Publicis reported net revenue of €13,965 million and described itself as the world’s largest advertising group by that measure, and Dentsu reported net revenue of ¥1,194.1 billion. These figures span different currencies and different metrics (total revenue versus net revenue), so there’s no clean single ranking, and Omnicom has since announced a proposed acquisition of Interpublic that will reshape this list going forward. More importantly for this article’s purpose: these are massive multi-discipline holding groups running traditional and digital advertising at global scale. They are not Klaviyo specialists or ecommerce retention agencies, and comparing them to a firm like Retention Side or Domaine is comparing different categories of business entirely.
What big companies use Klaviyo?
Several well-known consumer brands have published results using Klaviyo, based on the platform’s own case-study library. DKNY manages more than 1.2 million active subscribers and cut email sends 24% year over year while increasing click rate 8% through better segmentation and flows. Princess Polly consolidated email, SMS, app, and retail data into Klaviyo and saw Klaviyo-attributed revenue grow 2.8 times year over year in Q3 2025, with flows driving more than 60% of that revenue. Alessi grew email revenue 1,800% between 2019 and 2021 and generated 40% of Black Friday 2021 revenue from email. Marine Layer increased Klaviyo-attributed revenue 40.4% year over year, and Filson saw SMS revenue grow 3.5 times year over year, with Klaviyo generating 29% of total ecommerce revenue in 2024 year-to-date. Other brands referenced in Klaviyo’s customer library include Dollar Shave Club, ThirdLove, Tatcha, and Marc Fisher Footwear. One caveat worth noting: these are Klaviyo-published, platform-first case studies. Some reflect in-house execution rather than agency-led work, so don’t assume any specific agency was behind a given result unless that agency discloses it directly.
Which email marketing agency is the best?
There isn’t an objectively best email marketing agency, and any list claiming otherwise should be read skeptically. “Best” is a fit question: best for your revenue stage, your business model, your category, and how much of the retention system you want the agency to own versus execute. A brand doing $5M a year with a lean in-house team needs something different from a $50M multi-brand portfolio. Use the criteria in this article, verified partner status, business-metric case studies in a comparable context, the KPIs an agency reports by default, and their depth across channels beyond email alone, and you’ll land on the right fit faster than chasing a generic ranking.
The decision that actually matters
The right Klaviyo agency for your brand isn’t the one with the biggest badge or the longest client logo wall. It’s the one whose case studies, metrics, and process match the stage you’re at and the problem you actually have, whether that’s a deliverability issue quietly capping your revenue, flows that haven’t been touched in a year, or a channel mix that’s stuck on email alone when your customers are ready for more.
If you’re an established ecommerce brand evaluating whether your current setup is a channel or a system, that’s the conversation worth having before signing with anyone. Retention Side works with brands at exactly that inflection point, and reviewing your account against the criteria in this article is a reasonable first step regardless of who you end up choosing.