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Sendmux sets resource limits for each team. Free limits are fixed. Pro has no plan-level resource count limits. Provider quotas, safety controls, account health restrictions, and managed Amazon SES limits still apply.

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Free limits

Credentials include manual API keys across all key types and connected apps. Pending mailbox credential creation can also reserve capacity until it finishes. The API Keys page shows both credential classes in that total.

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Free limits are hard

A Free team cannot request a higher resource or managed-sending allowance. To get more capacity, delete an existing resource or upgrade to Pro. The app shows the upgrade path, and the Management API returns an upgrade message for a Free team.

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At the hard limit

Creation is blocked when usage reaches the full limit. A Free team must delete an existing resource or upgrade to Pro. API creation requests generate 409 limit_exceeded. Retrying the same request cannot succeed until capacity is available.

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Pro capacity

Pro has no plan-level limit on domains, mailboxes, sending accounts, webhooks, active credentials, team members, or provider-accepted recipients. It does not remove configured provider quotas, API rate limits, safety controls, account health restrictions, or managed Amazon SES safeguards.

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Email volume

Free teams can send to at most 50 accepted recipients per UTC day across all sending routes. A message with one To recipient and two CC recipients counts as three accepted recipients. Sendmux reserves capacity before accepting the message. Just before a queued message reaches its provider, Sendmux assigns that capacity to the current UTC day. If that day is already full, the queued message waits for the next UTC reset. In-flight recipients count towards the limit until their provider outcome is known, so concurrent sends cannot cross the cap. A reservation is not a usage charge. Outgoing usage is charged only for provider-accepted recipient occurrences. When a Free team has no daily capacity left, the HTTP Sending API returns 429 daily_limit_exceeded before queueing the message. SMTP returns 550 5.7.1 Daily sending limit reached. Upgrade to Pro to continue. The HTTP response includes Retry-After for the next 00:00 UTC reset. Messages already accepted into the queue wait instead of crossing the new day’s limit. Managed Amazon SES adds these safeguards: Pro does not have a Sendmux-wide outgoing volume cap. Managed sending limits, configured quotas, provider limits, and account health controls still apply.

Domains

Add and verify custom domains.

Sending accounts

Add providers and review managed sending limits.

Webhooks

Subscribe to delivery and mailbox events.

API keys

Create, rotate, and revoke scoped keys.

API errors

Handle limit_exceeded responses in API clients.