Updated February 24, 2026

TL;DR: Agencies scaling cold email need flat-fee platforms with unlimited accounts to avoid per-seat taxes that punish growth. Instantly offers unlimited sending accounts starting at $47/month, a deliverability network of 4.2M+ accounts, and agency white-labeling client portals. Apollo and Lemlist provide strong data and multichannel features but charge per seat or per inbox, compounding costs as you add clients. Smartlead is a solid flat-fee alternative with API-first design. Below I rank each by scalability, cost structure, and agency readiness.

Cold email at scale requires what I call "horizontal scaling." Instead of five inboxes each blasting 200 emails per day (which burns sender reputation fast), you need 30 inboxes each sending 30-50 emails safely. Only one pricing model supports this physics: flat-fee with unlimited accounts. Below I rank the best cold email software specifically for lead-gen and growth agencies, prioritizing economics, deliverability infrastructure, and multi-client workflows.

Why agency cold email requires a different tech stack

Agencies face a unique math problem. You need many inboxes but few users. A delivery team might manage dozens of client domains with over 100 sending accounts. In-house sales teams scale the opposite way (many users, fewer inboxes), so most CRMs and sales engagement platforms price per seat. That model collapses when your variable cost is mailboxes, not headcount.

The hidden costs of per-seat pricing models

Here is the margin trap. You sign a new client, add three sending domains with three inboxes each (nine total mailboxes). If your platform charges per user and you need six team members to access those accounts, you are paying for six seats. On Apollo's Professional plan, that is $474/month (6 users × $79/user on annual billing).

Contrast that with Instantly's Hypergrowth plan at $97/month. You get unlimited email accounts. Add 10 clients with 30 total inboxes and your cost stays $97/month. One G2 reviewer captured the economics:

"No limit on the number of emails you can add for one price. With Instantly I can drop 50 (or more) emails into one campaign and scale crazy fast." - Anthony V. on G2

This is not theoretical. Instantly's pricing page shows Growth at $47/month, Hypergrowth at $97/month, and Light Speed at $358/month, all with unlimited accounts and warmup included. Per-seat tools price you out of the very strategy (horizontal scaling) that protects deliverability.

Why unlimited accounts matter for deliverability

Deliverability is a volume game. Google, Yahoo, and Microsoft now enforce bulk sender rules requiring spam complaints under 0.3% and bounces under 2%. To stay under those thresholds at scale, you spread volume across more inboxes. If you need to send 1,000 emails per day, you should use 30-35 inboxes sending 30-35 emails each, not five inboxes sending 200.

Instantly cold email strategy recommends sending no more than 30 campaign emails per account per day after warmup. That means a 10-client agency sending 300 emails/day needs at least 10 warmed inboxes. Only flat-fee platforms let you add those inboxes without multiplying your software bill. This is why Instantly's unlimited account model is not a perk but a requirement for modern cold email physics.

Top cold email software for agencies: Ranked by scalability

I evaluated these platforms on five criteria: pricing model (flat vs per-seat), unlimited account support, warmup infrastructure, agency features (white-label, unified inbox), and ease of cancellation. Below are the top four, ranked by how well they handle multi-client scaling without punishing your margins.

1. Instantly: Best for unlimited scaling and flat-fee economics

Instantly is purpose-built for agencies managing 10–150+ inboxes on flat-fee economics: Growth $47/month, Hypergrowth $97/month, Light Speed $358/month, all with unlimited accounts and warmup included.

The platform gives you unlimited sending accounts on every plan, a private deliverability network of 4.2M+ warmup accounts, and built-in tools to manage multiple client workspaces from one login. The Unibox feature consolidates all client replies into a single feed, so your team handles 50 inboxes without 50 browser tabs.

Agency Pros:

• Unlimited accounts with zero marginal cost. Add 10 clients or 100 clients, your Hypergrowth plan stays $97/month. One reviewer explained:

"I love how easy Instantly is to set up, which significantly reduces the time I spend managing my B2B email marketing for my AI automation agency. The platform allows me to efficiently get in touch with hundreds, if not thousands, of business owners daily." - Shiv C. on G2

• White-label client portals. Set up a custom subdomain so clients log in at "reports.youragency.com" instead of "instantly.ai." You control branding and access permissions on Hypergrowth and above.
• Built-in warmup on 4.2M+ network. Turn on automated warmup and Instantly simulates natural engagement (opens, replies, read time) across its private network. No third-party tool required. Warmup uses slow ramp (day 1 send 2 emails, day 2 send 4, gradually increasing).
• SISR on Light Speed plan. Server and IP sharding and rotation automatically assigns dedicated IP pools and swaps flagged IPs, protecting reputation as you scale sending volume.
• SuperSearch lead database. 450M+ B2B leads with waterfall enrichment from 5+ providers. Export directly to campaigns without juggling separate data vendors.

Agency Cons:

• Each module (Outreach, SuperSearch, CRM) is priced separately. A full-stack agency setup runs $291–$391/month depending on credit tier. Still cheaper than per-seat alternatives at scale, but costs add up if you need all modules.
• Some advanced features (Priority AI and Follow-up AI in Hyper CRM) are marked "coming soon." Calling and SMS features are currently live.

Unlimited Claim: Verified. All Outreach plans include unlimited email accounts and warmup with no hidden caps. Fair use limits apply to daily sending per inbox (30 campaign emails recommended), driven by deliverability best practices, not platform restrictions.

Cancellation Policy: Self-serve. Cancellations take effect at the end of your billing cycle. No long-term lock-in.

Warmup & Deliverability: Instantly operates a private deliverability network with 4.2M+ accounts. Warmup is fully automated with read emulation, slow ramp, and weekday-only options. Inbox Placement tests validate health before you launch campaigns.

"Instantly has completely transformed our email outreach. The platform is super intuitive, easy to set up, and makes it simple to manage multiple domains and inboxes at scale" - Nathan D. on G2

For a visual walkthrough of campaign setup and account management, watch this full tutorial on YouTube.

2. Smartlead: Strong alternative for technical API users

Smartlead entered the market as a direct competitor to Instantly on unlimited accounts: Basic $39/month (monthly) or $32.50/month (annual), Pro around $94/month, with unlimited email accounts and warmup on all plans.

The platform includes unlimited warmup at no extra cost and offers a centralized Master Inbox for tracking conversations. Where Smartlead shines is its API-first design, making it easy to pipe data in and out of custom automation stacks.

Agency Pros:

• Unlimited accounts, flat pricing. The Basic plan at $39/month includes 2,000 active leads, 6,000 emails/month, and unlimited email accounts.
• White-label option with per-client add-on. White-labeling costs $29 per client. Pro plan includes one free client, then $29/month for each additional.
• No per-user limits. Your entire team can share one Smartlead account without additional seat charges.
• API and webhook-friendly. Strong documentation for developers who want to integrate with custom CRMs or build multi-tool workflows.

Agency Cons:

• UI can feel less polished for non-technical users compared to Instantly. If your delivery team includes junior staff who are not comfortable with APIs or complex settings, the learning curve is steeper.
• White-label is an add-on cost ($29/client), whereas Instantly includes it in Hypergrowth and above.

Unlimited Claim: Verified. Smartlead offers unlimited email accounts and warmup with no per-account fees.

Cancellation Policy: Monthly and annual plans with no long-term commitments. Cancel anytime.

Warmup & Deliverability: Automated warmup with dynamic IP addresses. Warmup is included in all plans at no extra cost.

For agencies comfortable with APIs and custom integrations, Smartlead is a solid, cost-effective alternative. For teams that prioritize UI simplicity and larger warmup infrastructure, Instantly edges ahead.

3. Apollo: Best for all-in-one data but costly to scale

Apollo is a sales intelligence powerhouse priced per seat: Basic $59/user/month, Professional $79/user/month, Organization $119/user/month (annual billing), with a database of 275M+ contacts and native CRM integrations.

If your agency also sells data enrichment or needs LinkedIn automation alongside email, Apollo's all-in-one approach is attractive. The problem for agencies is simple: per-seat pricing punishes team growth. Every user who touches client accounts costs $59-$119/month.

Agency Pros:

• Massive contact database. 275M+ leads with firmographic filters, intent data, and technographic signals. Export directly to sequences.
• Multichannel orchestration. Email, LinkedIn, and phone all in one platform. Strong for agencies running integrated campaigns.
• Deep CRM integrations. Native two-way sync with Salesforce, HubSpot, and other enterprise CRMs.
• Built-in warmup included. Apollo offers Email Warmup and Inbox Ramp Up features to improve deliverability.

Agency Cons:

• Per-seat model increases costs with team size. A six-person agency team on Professional plan pays $474/month (6 users × $79) before any add-ons or credits. Compare that to Instantly's $97/month for unlimited accounts.
• Credit system adds costs. Email credits are limited (10,000/month free tier, or calculated based on spend for paid accounts). Additional credits cost $0.20 each. Exporting verified mobile numbers consumes approximately 8 credits per number (around $1.60 per mobile when purchasing additional credits).
• Per-seat lock-in. Apollo doesn't allow reducing users mid-term, and annual subscribers must cancel 30 days before renewal.

Unlimited Claim: No. Apollo is per-seat. Adding more team members increases your monthly bill.

Cancellation Policy: Annual subscribers must cancel 30 days before term ends. Monthly subscribers cancel before 11:59 p.m. Pacific the day before next billing. No mid-term refunds for downgrades.

For agencies managing 30+ inboxes across multiple clients, Apollo's per-seat economics can become expensive quickly. Apollo is excellent if you need world-class data and multichannel features and can absorb per-seat pricing. For pure sending scale at flat fees, Instantly and Smartlead win.

4. Lemlist: Good for multichannel but expensive per seat

Lemlist pioneered the multichannel cold outreach category, priced per seat with inbox add-ons: as of February 2026, Email Pro $79/user/month and Multichannel Expert $109/user/month after a recent $10/user price increase.

The platform combines email sequences with LinkedIn automation (visits, connection requests, messages) and cold-calling steps. The platform includes Lemwarm, a built-in warmup tool that automates engagement between your accounts and other users on the network.

Agency Pros:

• True multichannel sequences. Build workflows that touch leads via email, LinkedIn, and phone in one campaign. The centralized inbox brings all conversations together regardless of channel.
• Lemwarm included free. Every paid plan includes access to Lemwarm for automated inbox warmup. No third-party tool required.
• Native CRM integrations. Salesforce, HubSpot, Chrome extension, and API access included on Email Pro and above.

Agency Cons:

• Per-seat pricing with inbox add-on costs. Email Pro gives you free inboxes per user, then charges for additional inboxes. Per-seat costs compound as you add team members to manage multiple clients.
• Credits don't roll over. Lemlist credits expire monthly. If you don't spend all your enrichment credits, they vanish at month-end.
• No white-label option. Unlike Instantly and Smartlead, Lemlist does not offer agency white-labeling in standard plans.

Unlimited Claim: No. Lemlist is per-user with additional per-inbox fees.

Cancellation Policy: Annual, quarterly, or monthly billing available. Longer commitments get discounts.

Warmup & Deliverability: Lemwarm is included free in all paid plans. Warmup spans 3-4 weeks with gradual ramp.

Lemlist excels if you need LinkedIn and calling automation in the same platform and can absorb per-seat costs. For agencies focused purely on email scale, flat-fee tools offer better economics.

Agency feature comparison: Pricing and accounts

Pricing & Account Model

Agency Features

Key Takeaway: Instantly and Smartlead are the only two platforms offering unlimited accounts on flat fees. Apollo and Lemlist charge per seat or per inbox, which compounds costs as you scale.

How to calculate your cost per meeting across platforms

Your agency's real metric is Cost Per Meeting (CPM): the total cost to book one qualified meeting. Software is just one input, but it is the easiest to control. Use this formula:

CPM = (Software Cost + Data Cost + Labor Hours × Hourly Rate) / Meetings Booked

Compare platforms using your current team size, inbox count, and monthly meeting volume. For example, a practical illustration from Instantly's agency comparison:

• A 6-person delivery team managing 10+ client mailboxes
• Instantly: $97/month on Hypergrowth (unlimited accounts)
• Apollo: $474/month just in seats (6 users × $79) before overages

The unlimited account model removes the classic per mailbox tax For agencies operating on 15-25% margins, software choice directly impacts profitability. Run this calculation with your current tool and inbox count. If your software cost per inbox exceeds $5-10/month, evaluate flat-fee alternatives.

Migration guide: Moving client campaigns without losing reputation

Switching platforms mid-campaign feels risky. You worry about losing sender reputation, breaking sequences, or dropping replies. Here is how to migrate safely in three steps.

Step 1: Export data and pause campaigns

• Download contact lists with custom fields, tags, and campaign status.
• Copy email templates, subject lines, and sequence logic into a doc.
• Export reply history or screenshot key threads.
• Save performance reports (open rates, reply rates, meetings by campaign).
• Pause active campaigns 24–48 hours before migration to avoid mid-transition sends.

Step 2: Connect domains and start warmup

• Sign up and connect sending domains to your new platform.
• Enable warmup immediately. Do NOT launch campaigns yet.
• Instantly's warmup uses slow ramp: Day 1 send 2 emails, Day 2 send 4 emails, Day 3 send 6 emails and so on.
• Warm for at least 2 weeks. Many teams see stable placement after 2–4 weeks on Google Workspace and Microsoft 365.
• You can begin small-scale cold outreach (10-20 emails per day) after 14 days if bounce rates stay under 1% and open rates exceed 35%, but full volume (30 emails per inbox per day) requires the complete 4-week ramp.

"Instantly has been an excellent experience from start to finish. The platform is extremely easy to set up, fast, and reliable for outreach. Their support team responds quickly and clearly, and they helped me resolve my issue immediately." - A Ta on Trustpilot

For a visual walkthrough of domain setup, watch this setup tutorial on YouTube.

Step 3: Recreate sequences and A/B test

• Import contact lists via CSV.
• Configure Campaign Options: daily send limits (start 20–30/day per inbox, ramp to 50 after two weeks), send windows during business hours (8:30–10:30 a.m., 1–3 p.m. local time), and inbox rotation to avoid fingerprinting.
• Launch one test campaign per client. Monitor inbox placement and reply rates for the first week.
• Adjust copy, send limits, or list quality as needed.

Conclusion: Choose the tool that protects your margins

Your software stack should scale with results, not headcount or inbox count. Per-seat pricing models punish you for the very strategies (horizontal scaling, multi-domain sending) that protect deliverability and book meetings. Flat-fee platforms like Instantly and Smartlead remove that tax.

If you run an agency managing 10+ client domains, the math is clear. Instantly's Hypergrowth plan at $97/month with unlimited accounts, 4.2M+ warmup network, and white-label client portals delivers predictable costs and protection as you scale. Apollo and Lemlist offer strong features (data, multichannel) but charge per seat or per inbox, which compounds costs as you grow. Smartlead is a solid alternative if your team prefers API-first workflows.

Start a free trial of Instantly today. Connect three client domains, enable warmup, and launch one test campaign. Add 10 inboxes or 100 inboxes without adding a dollar to your software bill. That is how agencies scale profitably in 2026.

Frequently asked questions about agency cold email software

What is the safest way to warm up multiple inboxes for clients?
Use a platform with automated warmup on a large network. Turn on read emulation and slow ramp, warm for at least 2-4 weeks, and send no more than 30 campaign emails per day per inbox after warmup completes.

How do I avoid getting a client's domain flagged or blacklisted?
Apply horizontal scaling: spread volume across 20-30 inboxes instead of 5. Keep bounce rates under 2%, use verified contact lists, and monitor inbox placement weekly.

Can I manage multiple clients in one login without mixing data?
Yes. Instantly's Agency feature and Unibox let you create separate workspaces per client with shared access and isolated campaigns.

What is the real difference between per-seat and flat-fee pricing for agencies?
Per-seat tools charge for every user. A six-person team on Apollo Professional pays $474/month. Instantly charges $97/month total for unlimited inboxes on Hypergrowth regardless of team size.

How long does it take to migrate from another platform to Instantly?
Export data in 1–2 days, connect domains and start warmup (2–4 weeks for full ramp), recreate sequences in 1–3 days. Total timeline: 3–5 weeks for safe migration without reputation loss.

Key terminology for agency operators

Unified Inbox (Unibox): A single dashboard that aggregates replies from all client inboxes, so you handle 50 mailboxes without switching tabs or logins.

Spintax (Spin Syntax): Text variations inserted into email templates to avoid duplicate content detection by spam filters. Example: "Hi {FirstName}, I {noticed|saw|found} your post on LinkedIn."

SISR (Server & IP Sharding & Rotation): Instantly's Light Speed feature that assigns dedicated IP pools and automatically swaps flagged IPs. This protects sender reputation when you scale to high-volume sending.

Horizontal Scaling: The practice of adding more sending inboxes (each sending lower volume) instead of increasing volume per inbox. Required to stay under ESP spam thresholds.

Warmup Network: A pool of real email accounts that exchange automated engagement (opens, replies, reads) to build sender reputation before launching cold campaigns. Instantly's network includes 4.2M+ accounts.