---
title: "Outsourced SDR Services Compared: Four Agencies, Ranked by What They Publish | Stone Haven Capital Group"
description: "Callbox, SalesHive, Belkins and Martal publish meeting counts and ROI multiples. StoneHaven publishes 98% inbox placement and a 0.97% bounce rate across 231,347 sends."
canonical: "https://stonehaven.capital/alternative/outsourced-sdr-services"
last-updated: "2026-08-30"
---

> Callbox, SalesHive, Belkins and Martal publish meeting counts and ROI multiples. StoneHaven publishes 98% inbox placement and a 0.97% bounce rate across 231,347 sends.

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Comparison

# Outsourced SDR Services Compared: Four Agencies, Ranked by What They Publish

August 30, 202613 min read

Choosing an outsourced SDR provider means comparing meetings booked, pipeline influenced, a multiple on ROI and a percentage cut off the sales cycle. Every one of those figures sits at the end of a chain: meetings depend on replies, replies depend on emails landing in an inbox, and inbox placement depends on sending infrastructure that no vendor puts on a website.

That is where the comparison collapses. Callbox, SalesHive, Belkins and Martal Group all say they book qualified meetings and all show case studies, and none of them publish the layer that fails first. This page compares the four on what they actually publish, and gives you the questions that separate a provider measuring its own delivery from one hoping.

Across every campaign StoneHaven runs, bounce stays between 1% and 3% and unsubscribes between 0% and 0.24%, with 98% inbox placement from the first send at 100,000+ emails a month.

## What outsourced SDR services actually deliver

Outsourced SDR services come in three shapes, each priced and measured differently, and the difference decides what you can hold a provider to.

Rented headcount. You pay for named sales development representatives working your account against a daily touch quota. The provider bills for capacity, so if the reps dial 250 times a day and book nothing, you still bought 250 dials. SalesHive is explicit about this model and prices on tiers of 150+, 250+ or 500+ daily touches. It fails on ramp and turnover.

Per-appointment. You pay for meetings that clear an agreed qualification bar. It reads as low risk, until the bar itself becomes the negotiation and a provider under quota starts widening it.

Managed sending programs. The provider builds and owns dedicated outbound infrastructure, runs the campaigns, handles replies and is measured on booked calls at a stated monthly send volume. The unit of accountability is the program rather than the headcount, and the thing that breaks it is deliverability. Any provider running this model should be able to hand you its placement and bounce numbers without a call.

## The four axes that decide an outsourced SDR program

Each axis below is checkable before you sign, and each one sits upstream of the meeting count everyone advertises.

| Axis | What to ask for | Why it decides the outcome |
|---|---|---|
| Send capacity | Monthly and daily send ceiling per client program | Booking rate, not reply rate, sets the volume you need. At 4% reply, 20% positive and 20% booking, 300 calls a month takes roughly 190,000 sends |
| Inbox placement | Placement rate at a stated monthly volume | An email in spam cannot reply. Placement is the first multiplier in the chain |
| Bounce rate | Bounce percentage on the last three programs | An unverified B2B list bounces 15-20%. A verified one lands near 1%. Bounce is the cleanest read on list hygiene |
| Reply rate with denominator | The percentage and the send count behind it | "4% reply" on 900 sends is noise. On 200,000 sends it is a result |

Infrastructure sits underneath all four. High-performing outbound runs on dedicated sending domains isolated from the client's corporate domain, mailboxes warmed before volume, and SPF, DKIM and [DMARC](https://www.rfc-editor.org/rfc/rfc7489) authenticated on every one. It diversifies across Google Workspace, Microsoft and private SMTP, because Google scores the domain while Microsoft scores the tenant and IP, and a single-provider bet means one filter change can take a program offline. [Google's sender guidelines](https://support.google.com/a/answer/81126) and [Microsoft SNDS](https://sendersupport.olc.protection.outlook.com/snds/) let a sender monitor this directly, so a provider claiming placement it cannot see chose not to instrument it.

## Callbox, SalesHive, Belkins and Martal compared on published data

Every figure in the table below came off the vendor's own site in August 2026. Where a cell reads "does not publish", the figure was absent from the public site, which means a buyer cannot verify that axis before signing.

|  | StoneHaven | Callbox | Belkins | Martal Group | SalesHive |
|---|---|---|---|---|---|
| Core model | Managed sending program, booked-call accountability | Multi-channel lead gen, appointment setting, outsourced SDR teams | Appointment setting and lead research | Outsourced sales team, lead gen plus closing support | Rented SDR headcount on an in-house platform |
| Primary channels | Email-first, with cold calling and cold DM layered on | Phone, email, LinkedIn, digital, events and webinars | Email and LinkedIn | Email, LinkedIn, cold calling | Phone and email |
| Published scale | Up to 250,000 sends a month per client program, up to 10,000 a day | 20+ years, 15K+ companies served, 10K+ campaigns, 60+ countries | Since 2017, 1,000+ clients, $2B+ client revenue generated | 15 years, 200+ onshore sales executives, 2,000+ teams served | 150+, 250+ or 500+ daily touches per tier |
| Send capacity published | Yes, 250,000/month per program | Does not publish | Does not publish | Case studies show per-client totals from ~10K to 364K emails | Touch quota only, no email send ceiling |
| Inbox placement published | Yes, 98% from first send at 100K+/month | Does not publish | Does not publish | Does not publish | Does not publish |
| Bounce rate published | Yes, 1-3% floor across every campaign; 0.97% on a 231,347-send workspace | Does not publish | Does not publish | Does not publish | Does not publish |
| Reply rate with denominator | Yes, 11.94% on 183,524 sends; 4.96% on 231,347 sends | Does not publish | Does not publish | Reply counts shown as absolute numbers, not rates | Does not publish |
| What they publish instead | Denominated campaign data plus program pipeline outcomes | 3x pipeline growth in 90 days, 30% appointment-rate increase, 40% shorter sales cycle, 2-week time-to-first-lead | 100-400+ appointments a year, up to 18,000 prospects, 200 sales-qualified meetings, 10-30 closed deals | Monthly funnel of 3,000-5,000 prospects, 20-30 qualified leads, 5-15 flipped leads | Month-to-month terms, no setup fee, US-based or offshore SDR options |
| Pricing | Scoped to program, quoted on a call | Quoted on a consultation | Quoted on a consultation | Four tiers, flat monthly fee, two with sales commission, quoted on request | One flat monthly fee, quoted on a 30-minute call |

In one 10-day cold-email run of 183,524 sends, StoneHaven held an 11.94% reply rate with 46% of replies flagged interested and a 2.82% bounce rate.

Callbox is the broadest of the four on geography. Twenty years of operating history, 15,000+ companies served and delivery teams across the US, UK, Australia, Singapore, Malaysia, Hong Kong and Colombia is real coverage, and cross-border programs are hard to staff. Their published metrics are all downstream outcomes: 3x average pipeline growth inside 90 days, a 30% lift in appointment rates, a 40% cut in sales cycle length. A buyer cannot audit any of them, because the site carries no send volume, placement or bounce figure to trace them back to.

Belkins publishes the most aggressive commercial framing in the set: $2B+ in revenue generated for clients since 2017, 1,000+ clients, and a stated yearly band of 100 to 400+ qualified appointments. That band is useful, because it is a commitment shape rather than a hero number. What sits behind it is not shown: no team size, no monthly send volume, no deliverability data anywhere on the site.

Martal Group comes closest to showing its work. Their case studies carry per-client email totals ranging from roughly 10,000 to 364,000 sends, which is more raw campaign data than Callbox or Belkins put in public. The gap is that replies appear as absolute counts rather than rates, so you can read the effort without reading the efficiency. Their 200+ onshore sales executives across North America, the EU and LATAM is a genuine strength for buyers who want a named human on the account.

SalesHive is the most honest of the four on commercial terms. Month to month, no setup fee, no long-term contract, and a plainly stated choice between US-based and offshore SDRs. For a buyer burned by a twelve-month agreement, that is worth more than a case study. Their measurement unit is the touch quota, and a touch is an input. Nothing on the site says what share of emailed touches reached an inbox.

The pattern across all four follows the shopping behaviour. Buyers ask about meetings and ROI multiples, so that is what goes on the website, while the deliverability layer that decides whether those meetings ever exist stays unmeasured in public.

## What the silence on deliverability costs a buyer

Take a program at 100,000 sends a month. At 98% inbox placement and 1% bounce, roughly 97,000 emails reach a human. At 70% placement, a level a shared or unwarmed setup hits easily, about 69,000 do.

Same list, same copy, same reps, same invoice, and 28,000 conversations gone before a word was read. Every downstream metric the vendor reports comes back smaller without ever naming the cause.

Rates without send counts behind them are the other half of the problem, which is why the [reply and bounce benchmarks pinned to real send counts](https://stonehaven.capital/blog/cold-email-reply-bounce-benchmarks-with-denominators/) are worth reading before you accept any vendor percentage. The bounce number is the cheapest audit a buyer can run. An unverified B2B list bounces 15-20%; a properly verified one lands near 1%.

Catch-all domains make up 20-30% of a typical B2B list and most verification tools guess on them, which is how a list that "passed validation" still burns a domain in week two. [M3AAWG's sender best practices](https://www.m3aawg.org/published-documents) treat list hygiene and authentication as the baseline of legitimate bulk sending.

Two anonymized programs show what the difference looks like at the pipeline end.

A mid-market B2B SaaS company was sending 8,000 emails a month from a single corporate inbox at a 1.1% reply rate, booking 3 or 4 meetings a month, and watching its main domain drift into spam.

Rebuilt onto 120,000 emails a month across 25 isolated domains and 600 mailboxes diversified across three infrastructure types, the program held 98% inbox placement from the first send and spam under 0.3%.

The steady-state monthly funnel ran 120,000 sent to 3,000 replies to 360 positive to 72 booked meetings, with about 70% showing.

Bounce ran between 1% and 3% throughout, and six months produced 432 qualified meetings and $1M+ in new ARR at roughly $24K ACV.

A precision-agriculture SaaS platform with 8,000+ grower accounts mapped 18,000 ICP accounts and enriched 64,000 decision-makers.

The program then ran ~380,000 emails across 120 domains and 360 mailboxes in 90 days at a 0.8% bounce rate.

That produced a 5.6% reply rate, roughly 4,300 positive replies, 2,150 booked meetings and 165+ qualified cooperative and distributor accounts.

Average contract value moved from $350K to $1.09M, and the influenced pipeline figure was $36M+ in 90 days.

## Engagement models, and why price is not the axis

None of the five providers on this page publishes a rate card, so a price comparison is not available to anyone, including StoneHaven. What differs is the shape of the commitment.

SalesHive quotes one flat monthly fee on a 30-minute call, runs month to month, and prices annual lower per month. Martal Group publishes four tiers, two of which add a sales commission on top of the flat fee. Belkins and Callbox both route to a consultation, and StoneHaven scopes pricing to the program and quotes it on a discovery call.

Read the commission tiers carefully. A provider taking a percentage of closed revenue is aligned with your outcome, and their team also touches your deals, which some sales leaders will not allow. Month-to-month terms cut your downside and cut your runway: warmed infrastructure comes before volume, so cancelling in month two means paying for the build and leaving before it compounds. The [outsourced SDR versus in-house breakdown](https://stonehaven.capital/alternative/outsourced-sdr-vs-in-house/) works through ramp, cost and ceiling on the same denominated basis.

## Where an outsourced SDR service is the wrong buy

Three situations where none of the providers on this page, including StoneHaven, is the right answer.

Your addressable market is under a few thousand accounts. If one rep can work every named account personally, relationship depth beats reach. The rough floor for an outbound program is around 30,000 reachable prospects.

Your buyer does not live in an inbox. Trades, local services, restaurants and hospitality buyers respond to phone and in-person channels far better than to email, so a calling-led provider fits them better. Callbox and SalesHive both run real phone operations.

Your offer is not yet closing. Outbound multiplies whatever your sales motion already does, so if held meetings are not converting today, more meetings produce more of the same at higher cost.

## Where StoneHaven fits

In one workspace, 231,347 emails sent to 77,115 leads carried a 0.97% bounce rate and a 4.96% reply rate, against the placement and bounce floor StoneHaven holds across every campaign.

On StoneHaven's own pipeline, the same model generated 445 sales calls in 25 days with Series A-C decision-makers, and 532 booked calls in one month that closed $379,000 in new revenue.

Best for: B2B SaaS, services, cybersecurity, industrial and manufacturing firms with mid-five-figure-plus deal sizes selling into founders, VPs of Sales, CROs and enterprise program owners.

If a booked-pipeline program built on published deliverability data fits how you want to grow, StoneHaven can walk through the numbers with you.

More depth on the meeting-first providers: the [appointment setting company breakdown](https://stonehaven.capital/alternative/best-appointment-setting-companies/) and the [Martal Group alternative page](https://stonehaven.capital/alternative/martal-group/).

If that description fits how you want to grow, [walk through the numbers with StoneHaven's team](https://stonehaven.capital/book/).

## FAQ

### What are outsourced SDR services?

Outsourced SDR services put the top of your sales funnel with an outside provider. The provider sources the list, runs the outreach across email, phone or LinkedIn, handles the replies and books qualified meetings onto your reps' calendars. Delivery splits into three shapes: rented headcount, where you pay for named SDRs and a daily touch quota; per-appointment models, where you pay for meetings that meet an agreed qualification bar; and managed sending programs, where the provider builds isolated email infrastructure and is measured on booked calls at a stated send volume. StoneHaven runs the third shape at up to 250,000 cold emails a month per client program, holding bounce between 1% and 3% across every campaign.

### How much do outsourced SDR services cost?

None of the four agencies in this comparison publish a dollar figure. SalesHive states one flat monthly fee quoted on a 30-minute call, with no setup fee and no long-term contract, priced on team model, channel mix and a daily touch volume of 150+, 250+ or 500+. Martal Group lists four tiers, two of which add a sales commission on top of a flat monthly fee, and quotes each on request. Belkins and Callbox both route pricing to a consultation, and StoneHaven quotes the same way. Price is not where these providers differ, so compare them on published performance data instead.

### Which outsourced SDR company is best for a B2B SaaS founder?

SalesHive, if your buyers answer the phone and you want US-based callers on a month-to-month agreement, because it publishes those terms plainly. Callbox, if you sell across borders, with 10,000+ campaigns since 2004 and delivery teams covering 60+ countries. If email volume is your constraint, the deciding question is inbox placement and bounce at a stated monthly send count, which none of the four publish.

### What deliverability data does StoneHaven publish?

StoneHaven publishes 98% inbox placement at 100,000+ emails a month, a 1-3% bounce floor and 0-0.24% unsubscribes across every campaign.

Campaign figures carry their send denominators: 231,347 emails to 77,115 leads at a 0.97% bounce rate and a 4.96% reply rate.

A single 10-day run of 183,524 sends held an 11.94% reply rate with 46% of replies flagged interested and 2.82% bounce. Open rates are deliberately not tracked, because tracking pixels hurt deliverability.

### Is outsourcing SDRs better than hiring in-house?

Hiring in-house buys you control and loses you time. A new sales development rep needs sourcing, onboarding and ramp before the first held meeting, and the cost carries whether or not the pipeline arrives. An outsourced program runs on infrastructure that already exists. StoneHaven's own outbound engine generated 445 sales calls in 25 days with Series A-C decision-makers, and booked 532 sales calls in a single month that closed $379,000 in new revenue. In-house wins when your market is small enough that a rep can work it by name and relationship depth matters more than reach.

### What should I ask an outsourced SDR agency before signing?

Ask five things in writing. How many sending domains and mailboxes will be dedicated to my program, and are they isolated from my corporate domain? What inbox placement and bounce rate did your last three programs hold, and at what monthly send volume? What is the denominator behind every rate you quoted me? What happens in the 48 hours after a domain starts burning? Who answers a reply that lands at 9pm? A provider that cannot answer the second question is asking you to buy an outcome it does not measure.

## Methodology

StoneHaven's figures come from campaign-dashboard data and published, industry-anonymized program case studies. The 183,524-send run covers a single 10-day campaign for one client in July 2025. The 231,347-send figure covers one workspace sending to 77,115 leads.

The 1-3% bounce and 0-0.24% unsubscribe range is observed across all campaigns run, and the 98% inbox placement figure applies to programs sending 100,000+ emails a month.

Booked-call figures (445 calls in 25 days, 532 calls in one month closing $379,000) come from StoneHaven's own internal sales pipeline rather than a client's. Pipeline and ACV figures in the two program examples belong to those clients and are reported as their influenced pipeline. Figures are never summed across dashboard panels or across programs. Open rates are not tracked and are not reported anywhere here.

Competitor data was read directly from callboxinc.com, saleshive.com, belkins.io and martal.ca in August 2026, including each company's pricing page where one exists. "Does not publish" means the figure was not present on the public site at that time; it does not mean the provider does not measure it internally. Vendor sites change, so verify current claims at the source before deciding.

Industry reference points on bounce rates, catch-all share and cost-per-rep context draw on [Google's sender guidelines](https://support.google.com/a/answer/81126), [M3AAWG published documents](https://www.m3aawg.org/published-documents) and [US Bureau of Labor Statistics wage data for sales representatives](https://www.bls.gov/ooh/sales/wholesale-and-manufacturing-sales-representatives.htm).

Last updated: August 2026 (Q3 2026).

Written by Sabo Nagy, Founder & CEO of StoneHaven. Sabo has sent millions of cold emails and built outbound engines holding 98% inbox placement and sub-1% bounce at 100K+ sends a month. [Author page](https://stonehaven.capital/author/sabo-nagy) - [X](https://x.com/MrColdEmail)

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