NimbusOS can sit above an existing sending platform or run campaigns through its own direct infrastructure. The five sending platforms supported as integrations are ReachInbox, Smartlead, Instantly, Lemlist, and Woodpecker. Each has different API strengths, different sync patterns, and different fit for different team sizes. This article covers what each integration does, when to choose each one, and how they interoperate with the NimbusOS campaign layer.
The Core Pattern: Platform as Executor
The NimbusOS design treats the sending platform as an executor, not a source of truth. NimbusOS owns:
- The campaign definition (sequence, targeting, throttles)
- The contact data and scoring
- The reply classification
- The analytics and attribution
The sending platform owns:
- The SMTP execution
- The warmup partner network (for ReachInbox)
- The inbox connections
When you configure a NimbusOS campaign with platform=smartlead, the platform creates the equivalent Smartlead campaign, pushes the sequence, enrolls the contacts, and polls for sends and replies. NimbusOS stays the control plane; Smartlead does the work.
ReachInbox
The default and most deeply integrated. Most agency fleets run on ReachInbox.
What the integration covers
- 28 API methods across campaigns, leads, accounts, warmup, analytics
- Webhook event ingestion for sent, delivered, opened, clicked, replied, bounced, unsubscribed
- Warmup analytics sync per inbox
- Account creation (IMAP account setup from NimbusOS UI)
- DNS check (with known limitations, see below)
Setup
Paste the ReachInbox API key in Integrations. The integration tests the key and lists available accounts. NimbusOS pulls all configured inboxes into EmailAccountHealth records.
Configure which ReachInbox workspace to sync (if your ReachInbox account has multiple workspaces).
Webhook configuration
ReachInbox pushes events to NimbusOS via webhook. Configure the NimbusOS webhook URL in ReachInbox's campaign-level settings. Events arrive within seconds of the underlying event in ReachInbox.
NimbusOS verifies webhook signatures using HMAC-SHA256. A valid signature is required; invalid signatures are logged and rejected.
Known limitations
Two ReachInbox API endpoints have upstream issues.
DNS check endpoint. check_account_dns() returns 404 or 500 on all known variants. Not used in production. NimbusOS runs DNS checks through its own DNSProvisioningEngine instead.
Analytics summary endpoint. GET /api/v1/analytics/summary does not exist. NimbusOS computes analytics from individual account and campaign endpoints.
Neither limitation affects campaign execution. They are noted for completeness.
Smartlead
Smartlead is a popular alternative sending platform. The integration pattern is similar to ReachInbox.
What the integration covers
- Campaign creation and management
- Lead list sync
- Account pool sync
- Analytics pull (opens, clicks, replies)
- Webhook event ingestion
Setup
API key-based. Paste the key in Integrations. The platform pulls accounts and existing campaigns.
Smartlead's API is less comprehensive than ReachInbox's, so some NimbusOS features (for example, granular warmup stage tracking) are approximated rather than exact when running through Smartlead.
When to choose Smartlead
If your team already runs campaigns in Smartlead and wants to add NimbusOS's contact data platform and reply intelligence on top without migrating sending infrastructure.
Instantly
Instantly is the other major alternative. Integration pattern identical to Smartlead.
What the integration covers
- Campaign creation and management
- Lead sync
- Account pool sync
- Analytics pull
- Webhook events (reply detection, unsubscribe)
Setup
API key-based. Similar to Smartlead.
When to choose Instantly
Same as Smartlead: team already invested in Instantly, wants NimbusOS on top. Instantly has a slightly different rate limit profile than Smartlead; for very high volume senders, one may fit better than the other depending on the provider's current capacity.
Lemlist
Lemlist is integrated for campaign sync. The integration is less deep than ReachInbox or Smartlead because Lemlist's API is more limited.
What the integration covers
- Campaign creation (limited to subject, body, step count)
- Lead sync (with Lemlist's specific lead object model)
- Analytics pull
- Reply events (via polling, not webhook)
Setup
API key-based.
Limitations
Lemlist does not support the full sequence model NimbusOS exposes. Complex branching sequences may not translate directly to Lemlist campaigns. For simple 3-step sequences, the integration works cleanly.
Woodpecker
Woodpecker is supported for campaign sync. Primarily useful for teams already running Woodpecker and wanting reply intelligence on top.
What the integration covers
- Campaign pull (read-only by default; write on request)
- Lead sync
- Reply ingestion for classification
Setup
API key-based.
Multi-Platform Fleets
Some workspaces run multiple sending platforms simultaneously. Common pattern:
- ReachInbox for the main cold outreach fleet
- Smartlead for a dedicated nurture fleet on a different domain pool
- Instantly for a client experiment
NimbusOS supports all three active integrations. Each campaign specifies its platform field at creation. Sends route accordingly.
Mixing platforms on one campaign is not supported. A single campaign uses one platform.
Platform Switching
Migrating an active campaign from one platform to another is not seamless. The general flow:
- Create a new campaign on the target platform with the same sequence.
- Pause the original campaign.
- Migrate the remaining contacts to the new campaign (via bulk enrollment).
- Archive the original.
Historical analytics stay with the original campaign. The new campaign starts fresh.
API Rate Limits
Each platform has its own rate limits that NimbusOS respects.
- ReachInbox: 5 requests per second
- Smartlead: 10 per second but with bursting
- Instantly: 5 per second
- Lemlist: 3 per second
- Woodpecker: 3 per second
NimbusOS's internal rate limiter matches these. For operations that bulk-apply to many contacts, the limiter queues requests and drains at the allowed rate.
Reply Routing
When a reply arrives at the sending platform, it is pushed to NimbusOS either via webhook (ReachInbox, Smartlead, Instantly) or pulled on polling (Lemlist, Woodpecker).
On arrival, the reply goes through:
- Reply intelligence classification (intent, sentiment, objection type).
- Auto-routing rules (Slack notification, sequence stop, etc.).
- Appear in the unified inbox with classification metadata.
This flow is the same regardless of platform. The unified inbox is a canonical view.
Account Health Parity
EmailAccountHealth records sync from each platform. The health fields (daily_send_limit, bounce_rate_7d, complaint_rate_7d) are pulled from the platform's analytics and kept in sync.
For some fields, the platform does not expose a direct equivalent, and NimbusOS computes approximations. For example, ReachInbox exposes a warmup stage directly; Smartlead does not, so NimbusOS computes the stage from cumulative warmup sends and days warming.
Switching a Campaign from ReachInbox to Smartlead
Sometimes teams want to migrate. Three steps.
- Add the Smartlead integration and sync accounts into NimbusOS.
- Pause the ReachInbox campaign.
- Create a new campaign with the same sequence, targeting the same contacts, with
platform=smartlead.
The new campaign will push down to Smartlead, enroll contacts, and start sending from Smartlead accounts. The ReachInbox campaign remains in NimbusOS as a historical record.
Troubleshooting
"Webhook events are not arriving from ReachInbox"
Check the webhook URL in ReachInbox settings. The URL should be https://getnimbusos.com/api/webhooks/reachinbox/. Also verify the webhook signature secret matches what NimbusOS expects.
"Smartlead campaign was created but it is missing leads"
The lead sync is asynchronous and can take 1 to 2 minutes after campaign creation. If leads are still missing after 5 minutes, check the integration log for errors.
"Instantly account shows healthy but no sends are happening"
The Instantly campaign might be paused on the Instantly side. Log into Instantly directly and verify the campaign status there. NimbusOS cannot always force-resume campaigns on external platforms.
"Lemlist sequence translation is wrong"
Lemlist does not support all NimbusOS sequence features. Conditional branching and sub-sequences flatten when translated. Use Lemlist only for simple linear sequences.
Frequently Asked Questions
Can I use NimbusOS without any external sending platform?
Yes, through BYOSMTP. Connect your own mailboxes via custom SMTP and run campaigns natively without an integrated platform. See BYOSMTP Configuration.
Does NimbusOS mark up send costs on any of these platforms?
No. You pay the sending platform directly. NimbusOS charges its platform fee independently.
What happens to campaigns if I disconnect a platform integration?
Active campaigns on the disconnected platform pause. In-flight sends complete. Reply sync stops (you will not see new replies until the integration is reconnected or a different platform takes over).
Can I use Woodpecker and ReachInbox in the same campaign?
No. One platform per campaign. For multi-platform sending, run parallel campaigns.
What to Read Next
Useful next pages after this one: Integration Overview for the full integration landscape, BYOSMTP Configuration for the platform-free sending path, and CRM Integrations for connecting campaigns to HubSpot and Salesforce.