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Use SMTP when an application can send mail through a relay but cannot call the Email API directly. Open Settings → SMTP for the host assigned to the current deployment.

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Connection settings

Do not guess or hard-code a generic SMTP hostname. Use the host displayed by the active Signal deployment.

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Sending requirements

Both the SMTP envelope sender, often called MAIL FROM, and the visible From address must use a domain verified in the same project as the API key. The key determines project scope and must be active, unexpired, and permitted to send.

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Configure a client

1

Create a dedicated key

Create an API key named for the SMTP caller and grant only Emails: Send.
2

Copy the connection values

Use Copy settings or copy host, port, encryption, and username individually.
3

Configure STARTTLS

Connect on port 587 and require the TLS upgrade. Do not silently fall back to an unencrypted connection.
4

Set sender identity

Configure MAIL FROM and From on a verified domain.
5

Send a controlled message

Deliver to an inbox you own and inspect the resulting message in Emails.

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Operational differences from REST

SMTP uses SMTP acceptance and reply codes instead of an HTTP 201 response. The relay does not provide the Email API’s JSON-body idempotencyKey field. If the SMTP caller can retry after an ambiguous connection failure, control duplicate risk in the caller or prefer REST for that workflow.

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Troubleshooting