Sendmux is a multi-provider transactional email API that handles both outbound sending and inbound mailboxes from one endpoint. Connect your own SMTP, Gmail OAuth, Microsoft 365, or Amazon SES. Receive on dedicated mailboxes that return parsed JSON. One bearer token, one OpenAPI 3.1 spec, two endpoints.
Outbound sending
Route through every provider you trust.
Connect SMTP, Gmail OAuth, Microsoft 365, or Amazon SES. Sendmux distributes traffic, enforces quotas, and reroutes around any failing provider. Every email leaves through your provider, your domain, your reputation.
Weighted distribution and auto-failover
Route a percentage of traffic to each provider, or define rules by domain or message type. When a provider returns 4xx or 5xx, traffic reroutes automatically and the failing provider gets disabled. Mix transactional and bulk on separate pools without cross-contaminating reputation.
Per-provider quotas
Per-second, minute, hour, and day caps. No upstream throttling at peak load.
HTTP and SMTP both
JSON over HTTPS or SMTP on port 587 with STARTTLS. Same API keys, same rate limits.
On Tue, Apr 23, 2026,
Jane wrote:<br/>
> earlier message…
> quoted thread…
Inbound mailboxes
Dedicated inboxes, clean JSON.
Every Sendmux mailbox is a real inbox on a real domain. Fully addressable, threaded, and searchable via API. Default to @myagent.mx or bring your own custom domain. Receive transactional, conversational, or threaded mail at agent speed.
Token-saving parsing on every message
Every message returns parsed JSON with an extracted_text field that drops quoted history, signatures, and tracking pixels before your agent reads a single token. Cut LLM tokens by up to 80% on inbound parsing.
Thread grouping
Conversations group automatically by Message-ID and References headers. RFC-2822 compliant.
Custom domains
SPF, DKIM, and DMARC. BIND zone file. Unlimited domains per team.
Platform
Multi-tenant by default.
Built for teams running email for their customers. Each team gets isolated providers, mailboxes, API keys, and metrics. Issue mailbox-scoped keys for one agent in one inbox, and nothing else. Cross-tenant access is impossible by design.
Team and mailbox-scoped API keys
Each API key belongs to a team or a single mailbox. Whitelist permission model with no implicit access. Owner, Admin, Developer, and Member roles cover the spectrum, gated by capability and never by hardcoded role name. Issue 10,000 mailboxes and pay nothing until they send.
Signed webhooks
HMAC-SHA256 per delivery. Exponential retry. A Server-Sent Events stream pushes the same events to live clients.
Usage-based billing
$0.15 per 1,000 emails for incoming and standard outgoing mail. $0.25 per 1,000 when sending through Sendmux SES. No per-seat or per-mailbox fees.