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Use SMTP when an existing application, plugin, or mail client already supports SMTP delivery.
The SMTP password is your smx_mbx_ send-only key, mailbox password, or owner-approved Sending-resource smx_agent_ token. Routing still follows that key or mailbox scope.
Managed Amazon SES is for transactional email only. Use your own provider for marketing, newsletters, or bulk promotional sends.

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Connection details

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Test with swaks

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Select delivery groups

Add one X-Sendmux-Delivery-Group header containing comma-separated group public IDs:
Replace the example IDs with your groups’ public IDs. Sendmux trims surrounding whitespace and deduplicates IDs, with a maximum of 50 groups. Empty entries and repeated occurrences of the header are rejected, including when separate occurrences contain the same value. A folded continuation belongs to the same header. The hint can only narrow the credential’s authorised providers. Shared-domain sending ignores it and retains its forced route. See Delivery groups for the scope rules. Unless shared-domain routing ignores the hint, invalid, unknown, foreign-team, or disallowed hints receive a permanent 550 response. Temporary service failures remain retryable. Sendmux removes the routing header before delivery. Use template variables in subjects and bodies to insert values from the selected account.

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SMTP vs HTTP

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What happens next

Successful SMTP sends appear in Logs like HTTP sends. Use the message ID, sender, recipient, status, provider, and timestamp to trace delivery.

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Acceptance during busy periods

When sending capacity is occupied, Sendmux can still accept your direct SMTP submissions using send-only keys or owner-approved Sending-resource tokens after it stores the message and reserves the required credits. SMTP 250 confirms acceptance for background sending, though delivery can still fail. Check Logs for the result. 452 4.3.1 Insufficient credits. means the message was not accepted.

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Free daily limit

Free teams can send to 50 accepted recipients per UTC day across SMTP and HTTP. Sendmux reserves capacity before returning 250, so concurrent SMTP sessions cannot cross the cap. When no daily capacity remains, SMTP returns 550 5.7.1 Daily sending limit reached. Upgrade to Pro to continue. The message is not queued. Retry after 00:00 UTC or upgrade the team to Pro.

Send by HTTP

Use the API for new integrations and batch sends.

API keys

Create the credentials used as SMTP passwords.

Delivery logs

Search and export SMTP delivery results.