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Delivery groups let you route a sending key or mailbox through a named set of providers. Use them to separate traffic by product, region, customer, or risk profile.

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How routing works

The key or mailbox you authenticate with defines the providers available to you. An optional per-message delivery-group hint narrows that selection. Groups have no activation setting. Their availability depends on eligible member providers, including provider status and sending limits. An empty eligible pool never falls back to providers outside the requested groups.

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Restrict one message

For HTTP, set delivery_group to one group public ID or a nonempty array in the send body. In a batch, set it on each message. For SMTP, use one X-Sendmux-Delivery-Group header with comma-separated public IDs. Sendmux trims surrounding whitespace, removes duplicate IDs, and accepts at most 50 groups. Empty entries are invalid. SMTP also rejects repeated occurrences of the header and strips the header before delivery. HTTP callers must use the dedicated field; the header is reserved in custom_headers. The credential’s scope determines how the hint is applied: Shared-domain routing takes precedence, even if the hint is malformed. Otherwise, Sendmux validates syntax, credential scope and group subset, then team-owned existence. A valid group ID from another team is treated as unknown. Routing failures use these codes: Batch responses retain errors at each message’s input index, including when every message fails routing. Temporary service failures remain retryable. Delivery logs expose the applied public IDs in nullable delivery_group. Absent, ignored, and historical hints have null. Later status updates preserve the applied groups.

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Create a group

1

Open Accounts

Open Accounts, then select Groups.
2

Create the group

Add a group name. You can edit the name and description later.
3

Add providers

Edit a provider and choose the delivery group in Advanced Settings.
CSV imports can create a group and assign an account in one step with the group_name column. See Sending accounts.

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Route a sending key

Create or edit a sending key and choose Delivery group(s) under sending scope. Select one or more groups. If none of the selected groups has an eligible provider, sends through that key fail until a provider is added or becomes eligible again.

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Route a mailbox

Use a mailbox’s send scope to bind it to a delivery group. Every API or SMTP send authenticated as that mailbox follows the mailbox scope.

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Delete a group

Deleting a group moves its providers to No group. Sendmux warns when API keys depend on the group so you can update their send scope first.

Sending accounts

Add providers and tune quotas, sender defaults, and weights.

API keys

Create sending keys and choose their send scope.

Mailboxes

Scope mailbox sending to a selected provider group.

Send by HTTP

Send messages with a key that already has the right route.