Sendmux
Sendmux is an email API that gives each AI agent its own real inbox with send, receive, routing, and events over one surface.
Sendmux attacks the week-two problem in agent builds: Gmail hands your model a wall of HTML, and you need clean text plus a way to reply. Stripped signatures and quotes, JSON with links and headers, and per-event metering that never rounds up to blocks of 1,000 are wins you can measure on the invoice — 1,000,000 connected recipients lands at $507/mo including the $7 team fee. It's a developer surface with a control-plane dashboard, not a mailbox. If the inbox is an afterthought in your stack, Resend or SendGrid will do; pick Sendmux when receiving, routing across your own providers, and MCP-exposed mail tools are the point.
Verified 48m ago · liveness 74/100 · cite: rightaichoice.com/tools/sendmux
- Agent developers who need a real inbox their agent can read, route, and reply to without an HTML-to-text layer
- Teams running support triage, scheduling, or monitoring agents that receive and send at volume
- Builders who want to bring their own sending providers and shift traffic between them without code changes
- Claude Code, OpenClaw, Codex, or LangChain stacks that want mail tools exposed over MCP
- Anyone who wants a visual email client or GUI-first inbox experience — the dashboard is a control plane, not a mailbox
- Bulk promotional or marketing campaigns; the managed sending path is transactional-shaped and governed by fair usage
- Non-technical teams looking for a no-code email tool
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Skip Sendmux if you need a visual email client, run bulk promotional campaigns, or want a no-code tool — it's a developer API with a control-plane dashboard and a transactional-only managed sending route.
Every accepted To, CC, and BCC recipient on an outbound send is billed separately at $0.50 per 1,000 connected, so a one-message blast to 20 people costs 20 events, not one.
Sendmux fits teams with real agent mail volume but no interest in per-seat billing: Pro is $7/team/mo plus usage, and 1,000,000 connected recipients runs about $507/mo with no per-mailbox fees. That lands above a raw Amazon SES pipe, which is why cost-first bulk senders should look there, and under per-seat platforms like SendGrid once your agent count grows. Enterprise is custom-priced for dedicated infrastructure and SLAs.
In short
Sendmux — Sendmux is an email API that gives each AI agent its own real inbox with send, receive, routing, and events over one surface. Best for Agent developers who need a real inbox their agent can read, route, and reply to without an HTML-to-text layer, Teams running support triage, scheduling, or monitoring agents that receive and send at volume, Builders who want to bring their own sending providers and shift traffic between them without code changes. Free to start; paid plans from $7/mo.
What's new in Sendmux
Checked todayAcross the latest 5 updates: 2 feature updates and 3 changelog entries.
Provider tags extended to From, Reply-To and Sender display names
Template variable provider tags now work in From, Reply-To and Sender display names, including encoded names. Subject and supported body substitution are unchanged, and other headers remain excluded.
Inbox bulk-change progress bar and confirmation flow fixed
The Inbox bulk change progress bar now persists until the conversation list reflects the update, then clears with a confirmation. Failed and partial changes remain visible for review.
MCP sign-in fixed for clients with unsupported grant types
MCP sign-in now works for clients whose published OAuth metadata lists grant types the authorization server does not offer, such as Claude, instead of refusing the connection.
Inbound charge fix, 254-char email limit, mailbox activation gating
Fixed stuck inbound email usage charges, capped signup email addresses at 254 characters, stopped dashboard requests from loading on connection failure, and gated new mailbox activation on verified mail access.
Fixed Codex sign-in rejection on mismatched callback port
Codex sign-in requests are no longer rejected when the local callback port differs from the port in the client's published OAuth metadata.
Viability Score
How well maintained and how widely used is Sendmux? Built from what the vendor actually publishes (docs, changelog, tutorials, integrations, pricing), whether the site is live, and how much real users discuss it. How we calculate this
Last calculated: October 2026
How we score →Key Features
- Create agent mailboxes on verified domains or the shared myagent.mx domain
- Inbound signature and quoted history stripped by default via strip_signature and strip_quotes
- Clean JSON responses with links, headers, and attachment metadata on request
- Outbound email over HTTP with Idempotency-Key support for safe retries
- SMTP sending using the same mailbox credential as the Bearer token and password
- Send text, HTML, X-headers, and up to 10 attachments per message
- Multi-provider routing with percentage weights and second/minute/hour/day quota windows
- Delivery groups to keep separate provider pools for failover
- Live events via signed webhooks, SSE stream, and typed change polling
- Batch message actions accepting up to 100 message IDs
- Attachment upload before send, referenced by ID at send time
- Mailbox file storage: list and read uploaded files via API
- Scoped API keys separating sending, management, and mailbox access
- SDKs for TypeScript, Python, Go, PHP, and Ruby plus the @sendmux/cli
- Hosted MCP endpoint at mcp.sendmux.ai with OAuth sign-in for agent clients
About Sendmux
Sendmux is an email API built for AI agents, so each agent gets a real inbox rather than a bolted-on Gmail workaround. Mailboxes live on a verified domain you control or on the shared myagent.mx domain, and inbound mail arrives shaped for LLM consumption: signatures and quoted history are stripped by default (strip_signature, strip_quotes), and you can request clean JSON with links, headers, and attachment metadata so agents parse structured content instead of burning tokens on raw HTML. Outbound runs over HTTP or SMTP using the same mailbox credential as a Bearer token and SMTP password, with Idempotency-Key support for safe retries, X-headers, and up to 10 attachments per send; batch message actions accept up to 100 IDs at a time. Routing separates it from send-only APIs: bind a mailbox key to all active providers, chosen providers, or a delivery group, then cap traffic with percentage weights and second, minute, hour, and day quota windows so a degraded provider sheds load without a code change. Events arrive as signed webhooks for delivery, receipt, spam, and tests, an SSE stream for live mailbox clients, or typed change polling for resync. Tooling covers TypeScript, Python, Go, PHP, and Ruby SDKs, a CLI (@sendmux/cli), a hosted MCP endpoint at mcp.sendmux.ai with OAuth sign-in, and documented LangChain and Vercel AI SDK integrations. Pricing is metered per event, not per seat: Free at $0/team/mo with fixed limits and a $1 usage credit, Pro at $7/team/mo plus prepaid usage, and Enterprise custom. On Pro, connected outbound is $0.50 per 1,000 provider-accepted recipients, managed sending $0.75 per 1,000, inbound $0.50 per 1,000 distinct mailbox deliveries, and storage $0.02 per GB-month — billed per event, never in blocks of 1,000. The audience is narrow: agent developers shipping autonomous systems that read, triage, and reply to mail without a human in the loop.
Behind the Verdict
Sendmux makes a specific bet: that an AI agent needs a real mailbox, not a sending API with a webhook bolted on. The evidence for that bet is in the inbound path. Signatures and quoted history are stripped by default via strip_signature and strip_quotes, and you can request links, headers, and attachment metadata as structured JSON — which is exactly what you'd otherwise write yourself, badly, as a Gmail parser. Batch message actions take up to 100 IDs at a time, so a triage agent can act on a whole queue rather than looping one message at a time. The routing layer is the part most competitors skip. You bind a mailbox key to all active providers, chosen providers, or a delivery group, then cap traffic with percentage weights and second, minute, hour, and day quota windows. That means a degraded sending provider sheds load without a code deploy — a real operational difference if you run deliverability-sensitive agents. Delivery groups keep separate provider pools for failover, and the deliverability view watches bounce rates, complaint rates, and provider health across multiple sending accounts. The integration surface is unusually agent-native for a product this young: SDKs for TypeScript, Python, Go, PHP, and Ruby, a CLI, a hosted MCP endpoint at mcp.sendmux.ai with OAuth sign-in, documented LangChain and Vercel AI SDK integrations, plus skills installable with npx skills add Sendmux/skills. If your stack is Claude Code, OpenClaw, or Codex, mail tools show up without writing a tool wrapper. Where it doesn't fit: the dashboard is a control plane, and there is no visual email client. The managed sending route runs on Amazon SES and is documented as transactional only, so marketing, newsletters, and bulk promotional sends have to go through your own connected provider. The Free plan is genuinely fixed — 2 mailboxes, 1 domain, 1 connected sending account, 1 webhook, 2 credentials, owner-only team membership, and 50 provider-accepted recipients per UTC day, with managed sending held to 2 per hour, and those limits cannot be topped up. Note also how billing counts: each accepted To, CC, and BCC recipient is a separate billable event, and one inbound email landing in two mailboxes counts as two deliveries. At high volume, $0.50 per 1,000 connected recipients sits well above a raw SES setup — you are paying for the inbox, routing, and event layer, not the raw pipe.
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Real-world workflow fit
Concrete scenarios for the personas Sendmux actually fits — and what changes day-one when you adopt it.
Start on Free, create an agent mailbox on your own verified domain, connect a LangChain or Vercel AI SDK agent with send_email, list_messages, and reply tools, and stream message.received events over SSE instead of polling.
Outcome: The agent reads clean JSON with signatures and quotes already stripped, replies over HTTP with an Idempotency-Key, and you upgrade to Pro only when you clear 50 provider-accepted recipients in a day.
Bind mailbox keys to delivery groups, set percentage weights across connected providers, and apply second/minute/hour/day quota windows so a degraded provider sheds traffic automatically; watch bounce and complaint rates in the deliverability view.
Outcome: Provider failover happens without a code change, and per-event metering at $0.50 per 1,000 connected recipients plus $0.02 per GB-month storage keeps the invoice tied to actual traffic.
Add the hosted MCP endpoint at mcp.sendmux.ai with OAuth sign-in, or install skills via npx skills add Sendmux/skills, then create a scoped API key that separates sending, management, and mailbox access.
Outcome: Mailbox, Sending, and Management tools appear in the agent without writing a wrapper, and scoped keys keep sending credentials away from management operations.
Use Cases
- Give your AI agent a dedicated address so it receives and answers customer inquiries without a human in the loop.
- Route incoming support mail to the right agent by content and sender rules, with signatures stripped to cut token spend.
- Send transactional receipts or notifications over HTTP or SMTP with idempotency keys so retries are safe.
- Watch deliverability, bounce rates, complaint rates, and provider health across multiple sending accounts from one dashboard.
- Stream live received, sent, and spam events to your agent over SSE for real-time decisions instead of polling loops.
- Use agent mailboxes as a file store: files arrive by email or upload, and agents list, filter, and read them on demand.
Limitations
- Sendmux is a developer email API, not a mail client, and the dashboard is functional but spartan.
- The managed sending route runs on Amazon SES and is documented as transactional-only, so marketing, newsletters, and bulk promotional sends have to go through your own connected provider.
- The Free plan has fixed limits that cannot be topped up: 2 mailboxes, 1 domain, 1 connected sending account, 1 webhook, 2 credentials, owner-only team membership, and 50 provider-accepted recipients per UTC day across all outbound routes, with managed sending held to 2 per hour.
- One active Free team per owner.
- Usage is billed per event, and each accepted To, CC, and BCC recipient counts separately, while one inbound email landing in two mailboxes counts as two deliveries.
- Higher managed daily limits on Pro require review, and dedicated infrastructure, priority support, volume pricing, and custom SLAs sit behind Enterprise.
as of 2026-10-09
Verification history
We have re-verified Sendmux 11 times since . Each pass re-reads the vendor's own pages and re-checks every listed field against that evidence; passes where nothing had changed are marked as such.
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-checked, vendor evidence unchanged
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
Showing the 6 most recent of 11 verification passes.
Free to cite with attribution — this page re-verifies continuously.
12-month cost
Project the real annual outlay, including the implied monthly cost when only an annual tier is published.
Vendor list price only. Add-on usage, seat overages, and contract minimums are surfaced under Hidden costs & gotchas.
Plans compared
For each published Sendmux tier: who it actually fits, and what it adds vs. the previous tier. Cross-reference the cost calculator above for projected annual outlay.
Free
$0/team/mo
Ideal for
Solo developer prototyping an agent mailbox who sends under 50 provider-accepted recipients a day and doesn't need teammates in the account
What this tier adds
Starting tier: $0/team/mo with a $1 usage credit, 2 mailboxes on 1 domain, 1 connected sending account, 1 webhook, 2 credentials, and a 'Email sent via Sendmux' footer on outbound
Pro
$7/team/mo + usage
Ideal for
Production agent teams past the Free caps who want unlimited mailboxes and domains without per-seat or per-mailbox fees
What this tier adds
Adds the $7/team/mo platform fee plus prepaid usage with manual and automatic credit top-ups, removes the outbound footer, and unlocks storage at $0.02 per GB-month and inbound at $0.50 per 1,000 deliveries
Enterprise
Custom
Ideal for
High-volume senders needing dedicated infrastructure, custom SLAs, or postpaid billing rather than prepaid credits
What this tier adds
Replaces public per-1,000 rates with volume-based custom pricing and adds dedicated infrastructure, priority support, and postpaid usage billing
Where the pricing makes sense
The company stage and team size where Sendmux's pricing actually pencils out — and where peers do it cheaper.
Sendmux fits teams with real agent mail volume but no interest in per-seat billing: Pro is $7/team/mo plus usage, and 1,000,000 connected recipients runs about $507/mo with no per-mailbox fees. That lands above a raw Amazon SES pipe, which is why cost-first bulk senders should look there, and under per-seat platforms like SendGrid once your agent count grows. Enterprise is custom-priced for dedicated infrastructure and SLAs.
Setup time & first value
How long it actually takes to get something useful out of Sendmux — broken out by persona, not the marketing-page minute.
Individual developers: roughly five minutes from signup to a first test send, following the Quickstart to create a team, add a mailbox, and send a test email. Teams wiring MCP or SDKs: about 20-30 minutes including OAuth sign-in and scoped key creation. Multi-provider routing setups with delivery groups and quota windows: plan an afternoon, since provider credentials and domain verification have
Switching to or from Sendmux
How to bring data in from common predecessors and how to get it back out — written for the switcher, not the buyer.
- →From Gmail-based agent hacks: create a mailbox on a verified domain or myagent.mx, then replace your Gmail parsing with the Mailbox API's strip_signature and strip_quotes JSON responses.
- →From Resend or SendGrid: move outbound to the Sending API over HTTP or SMTP, then keep your existing provider as a connected sending account and route through delivery groups.
- →From a raw Amazon SES setup: connect SES as a sending account and gain the inbox, routing, and event layer on top of the same pipe.
- →From polling loops to SSE: swap your periodic inbox fetch for mailbox.streamMailboxEvents filtered on message.received, or use typed change polling if a client needs to resync.
- →From hand-rolled LangChain tools: install langchain-sendmux and use SendmuxToolkit's send_email, list_messages, and reply instead of custom wrappers.
- ↗To Resend or SendGrid: export your sending accounts and recipients, then rebuild outbound against their sending API — you'll lose the inbound mailbox and routing layer.
- ↗To a raw Amazon SES setup: point your existing SES credentials at SES directly and drop the per-event layer, trading the inbox API for a lower per-message cost.
- ↗To a GUI email client: route agent mailboxes through IMAP-style SMTP settings so a human can read them, but expect manual triage instead of programmatic routing.
- ↗To an in-house implementation: re-create signature and quote stripping, multi-provider weighting, and signed webhook verification yourself — feasible, but it is exactly the work Sendmux exists to remove.
Integrations
Resources & Guides
Tutorials & Learning
YouTube returned 6 videos for “Sendmux”, and we withheld 6: 6 could not be judged, because “Sendmux” is a single word that other videos use for other things. We are showing none, because we could not prove any of them are about Sendmux.
Official links
Tools that pair well with Sendmux
Common stack mates teams adopt alongside Sendmux, with the specific reason each pairing earns its keep.
HEY Email
HEY Email is 37signals' rethink of email: a Screener vets first-time senders so only people you approve ever reach your Imbox.
Shortwave
Shortwave is an AI email assistant and native client where Claude 4.6 agents manage triage, drafting, scheduling, and search in your inbox.
Superhuman
Superhuman is AI email, docs, and a proactive Go assistant whose agents run inside Slack and your other work apps.
Featured Head-to-Head Comparisons
Sendmux vs Spider Cloud
Choose Sendmux if your AI agent needs dedicated email inboxes and autonomous email handling with clean JSON output to minimize LLM tokens. Choose Spider Cloud if your agent relies on real-time web data for RAG pipelines, with a Rust-powered engine and advanced unblocking. Both are purpose-built for agentic workflows but serve orthogonal data modalities.
Sendmux vs Presto Voice
Presto Voice and Sendmux are not competitors; they serve completely different domains. Choose Presto Voice if you operate a QSR chain seeking drive-thru voice AI with proven revenue lift. Choose Sendmux if you're building AI agents that need a purpose-built email API with clean data and usage-based pricing. There is no overlap.
Sendmux vs Temporal Ai
Choose Temporal AI if your primary need is durable, fault-tolerant orchestration for AI agents and multi-step workflows. Choose Sendmux if your agent requires dedicated email handling (send, receive, route) with clean structured data. Temporal is broader but Sendmux is purpose-built for email.
Alternatives to Sendmux
View all
HEY Email
HEY Email is 37signals' rethink of email: a Screener vets first-time senders so only people you approve ever reach your Imbox.
Shortwave
Shortwave is an AI email assistant and native client where Claude 4.6 agents manage triage, drafting, scheduling, and search in your inbox.
Superhuman
Superhuman is AI email, docs, and a proactive Go assistant whose agents run inside Slack and your other work apps.
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