> ## Documentation Index
> Fetch the complete documentation index at: https://www.courier.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> Install the Courier skill before writing code: `npx skills add trycourier/courier-skills`. It carries the verified SDK shapes and the rules you cannot get wrong.
> Authenticate every request with `Authorization: Bearer <API_KEY>`. A workspace has several environments and each has its own keys, which are plain `pk_` strings with no environment prefix. Start with Test.
> Send with `client.send.message` from the Node SDK (`@trycourier/courier` v7 and later, where the client is the default import). Reference a template by its `nt_` id or its alias.
> A send accepts a bare Elemental element list, but storing content on a template requires the top-level elements wrapped in a channel element.
> Templates and journeys can be built in the Courier app or created through the API. Either way they live in the workspace and are referenced by ID when you send.
> The hosted MCP server is https://mcp.courier.com. For a briefing on what Courier is and when to use it, read https://www.courier.com/llms.txt.
> Prefer the Guides tab for how-do-I questions and the Docs tab for how-does-it-behave questions. The API reference lives under /api-reference.

# Send email with Resend through Courier

> Connect Resend with an API key and from address, then override sender fields per send.

export const Doc = ({href, children, name, bare}) => {
  const label = children || name || href;
  if (bare) {
    return <a href={href}>{label}</a>;
  }
  return <a className="cx-endpoint" data-kind="doc" href={href}>
      <span className="cx-endpoint-label">{label}</span>
      <span className="cx-endpoint-method">DOC</span>
    </a>;
};

export const AppLink = ({href, children, name, bare}) => {
  const label = children || name || "Open in Courier";
  if (bare) {
    return <a href={href} target="_blank" rel="noreferrer">{label}</a>;
  }
  return <a className="cx-endpoint" data-kind="app" href={href} target="_blank" rel="noreferrer">
      <span className="cx-endpoint-label">{label}</span>
      <span className="cx-endpoint-method" aria-hidden="true">↗</span>
    </a>;
};

## Prerequisites

* <AppLink href="https://app.courier.com/signup">A Courier account</AppLink>
* [A Resend account](https://resend.com/signup)

## Setup

<Steps>
  <Step title="Create a Resend API Key">
    In Resend, open [API Keys](https://resend.com/api-keys). Create an API key and copy it.
  </Step>

  <Step title="Configure in Courier">
    In Courier, open the <AppLink href="https://app.courier.com/integrations/catalog/resend">Resend Integration</AppLink> page. Paste your API key into the "API Key" field.
  </Step>

  <Step title="Set From Address">
    Add a verified email address to the "From Address" field (e.g., `noreply@yourdomain.com`). Click "Add Integration" then "Save."
  </Step>
</Steps>

Addressing a recipient and sending are the same on every email provider, so they are documented once: <Doc href="/docs/integrations/email/overview#profile-requirements">profile requirements</Doc> and <Doc href="/docs/integrations/email/overview#send-to-a-recipient">send to a recipient</Doc>.

## Overrides

<Doc href="/docs/send/overrides#how-overrides-work">How overrides work</Doc> covers the two levels and which one wins. <Doc href="/docs/integrations/email/overview#channel-overrides">Email channel overrides</Doc> lists the fields every email provider takes.

A provider override changes what Courier sends to Resend's [/emails endpoint](https://resend.com/docs/api-reference/emails/send-email). Use one to set a field Courier does not support yet, or to replace a value Courier generates.

For example, add Resend tags:

<CodeGroup>
  ```javascript Node.js highlight={8-19} theme={null}
  const { requestId } = await courier.send.message({
    message: {
      template: "nt_01kx4h2jdafq8bk9aftxak4b40",
      to: {
        email: "sarah@acme-corp.com",
      },
      providers: {
        resend: {
          override: {
            body: {
              tags: [
                {
                  name: "environment",
                  value: "development",
                },
              ],
            },
          },
        },
      },
    },
  });
  ```

  ```python Python highlight={8-19} theme={null}
  response = client.send.message(
      message={
          "template": "nt_01kx4h2jdafq8bk9aftxak4b40",
          "to": {
              "email": "sarah@acme-corp.com",
          },
          "providers": {
              "resend": {
                  "override": {
                      "body": {
                          "tags": [
                              {
                                  "name": "environment",
                                  "value": "development",
                              },
                          ],
                      },
                  },
              },
          },
      },
  )
  ```

  ```bash cURL highlight={11-22} wrap theme={null}
  curl -X POST https://api.courier.com/send \
    -H "Authorization: Bearer $COURIER_API_KEY" \
    -H "Content-Type: application/json" \
    -d '{
      "message": {
        "template": "nt_01kx4h2jdafq8bk9aftxak4b40",
        "to": {
          "email": "sarah@acme-corp.com"
        },
        "providers": {
          "resend": {
            "override": {
              "body": {
                "tags": [
                  {
                    "name": "environment",
                    "value": "development"
                  }
                ]
              }
            }
          }
        }
      }
    }'
  ```

  ```ruby Ruby highlight={8-19} theme={null}
  response = courier.send_.message(
    message: {
      template: "nt_01kx4h2jdafq8bk9aftxak4b40",
      to: {
        email: "sarah@acme-corp.com"
      },
      providers: {
        resend: {
          override: {
            body: {
              tags: [
                {
                  name: "environment",
                  value: "development"
                }
              ]
            }
          }
        }
      }
    }
  )
  ```

  ```go Go highlight={10-21} theme={null}
  response, err := client.Send.Message(context.TODO(), courier.SendMessageParams{
  	Message: courier.SendMessageParamsMessage{
  		To: courier.SendMessageParamsMessageToUnion{
  			OfUserRecipient: &shared.UserRecipientParam{
  				Email: courier.String("sarah@acme-corp.com"),
  			},
  		},
  		Template: courier.String("nt_01kx4h2jdafq8bk9aftxak4b40"),
  		Providers: shared.MessageProvidersParam{
  			"resend": shared.MessageProvidersTypeParam{
  				Override: map[string]any{
  					"body": map[string]any{
  						"tags": []any{
  							map[string]any{
  								"name": "environment",
  								"value": "development",
  							},
  						},
  					},
  				},
  			},
  		},
  	},
  })
  ```

  ```java Java highlight={6-15} theme={null}
  SendMessageParams params = SendMessageParams.builder()
      .message(SendMessageParams.Message.builder()
          .to(UserRecipient.builder().email("sarah@acme-corp.com").build())
          .template("nt_01kx4h2jdafq8bk9aftxak4b40")
          .providers(MessageProviders.builder()
              .putAdditionalProperty("resend", JsonValue.from(java.util.Map.of("override", java.util.Map.of(
                      "body", java.util.Map.of(
                          "tags", java.util.List.of(
                              java.util.Map.of(
                                  "name", "environment",
                                  "value", "development"
                              )
                          )
                      )
                  ))))
              .build())
          .build())
      .build();
  SendMessageResponse response = client.send().message(params);
  ```

  ```php PHP highlight={8-19} theme={null}
  $response = $client->send->message(
    message: [
      'template' => 'nt_01kx4h2jdafq8bk9aftxak4b40',
      'to' => [
        'email' => 'sarah@acme-corp.com',
      ],
      'providers' => [
        'resend' => [
          'override' => [
            'body' => [
              'tags' => [
                [
                  'name' => 'environment',
                  'value' => 'development',
                ],
              ],
            ],
          ],
        ],
      ],
    ],
  );
  ```

  ```csharp C# highlight={9-28} theme={null}
  SendMessageParams parameters = new()
  {
      Message = new()
      {
          To = new UserRecipient { Email = "sarah@acme-corp.com" },
          Template = "nt_01kx4h2jdafq8bk9aftxak4b40",
          Providers = new Dictionary<string, MessageProvidersType>()
          {
              {
                  "resend",
                  new()
                  {
                      Override = JsonSerializer.Deserialize<Dictionary<string, JsonElement>>(
                          """
                          {
                            "body": {
                              "tags": [
                                {
                                  "name": "environment",
                                  "value": "development"
                                }
                              ]
                            }
                          }
                          """
                      ),
                  }
              },
          },
      },
  };

  var response = await client.Send.Message(parameters);
  ```

  ```bash CLI highlight={5} wrap theme={null}
  courier send message \
    --api-key "$COURIER_API_KEY" \
    --message.to '{"email": "sarah@acme-corp.com"}' \
    --message.template nt_01kx4h2jdafq8bk9aftxak4b40 \
    --message.providers '{"resend": {"override": {"body": {"tags": [{"name": "environment", "value": "development"}]}}}}'
  ```

  ```text MCP theme={null}
  With Courier MCP, send my template to sarah@acme-corp.com and set a Resend field Courier does not expose.
  ```
</CodeGroup>

Courier merges everything inside `message.providers.resend.override.body` into its generated request body. You can also override config values:

```json theme={null}
{
  "message": {
    "providers": {
      "resend": {
        "override": {
          "config": {
            "apiKey": "<your override API key>",
            "fromAddress": "alternate-sender@yourdomain.com"
          }
        }
      }
    }
  }
}
```

### Attachments

To include an attachment, add an `attachments` array in the override. File content must be base64-encoded.

<CodeGroup>
  ```javascript Node.js highlight={8-18} theme={null}
  const { requestId } = await courier.send.message({
    message: {
      template: "nt_01kx4h2jdafq8bk9aftxak4b40",
      to: {
        email: "sarah@acme-corp.com",
      },
      providers: {
        resend: {
          override: {
            attachments: [
              {
                filename: "billing.pdf",
                contentType: "application/pdf",
                data: "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh",
              },
            ],
          },
        },
      },
    },
  });
  ```

  ```python Python highlight={8-18} theme={null}
  response = client.send.message(
      message={
          "template": "nt_01kx4h2jdafq8bk9aftxak4b40",
          "to": {
              "email": "sarah@acme-corp.com",
          },
          "providers": {
              "resend": {
                  "override": {
                      "attachments": [
                          {
                              "filename": "billing.pdf",
                              "contentType": "application/pdf",
                              "data": "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh",
                          },
                      ],
                  },
              },
          },
      },
  )
  ```

  ```bash cURL highlight={11-21} wrap theme={null}
  curl -X POST https://api.courier.com/send \
    -H "Authorization: Bearer $COURIER_API_KEY" \
    -H "Content-Type: application/json" \
    -d '{
      "message": {
        "template": "nt_01kx4h2jdafq8bk9aftxak4b40",
        "to": {
          "email": "sarah@acme-corp.com"
        },
        "providers": {
          "resend": {
            "override": {
              "attachments": [
                {
                  "filename": "billing.pdf",
                  "contentType": "application/pdf",
                  "data": "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh"
                }
              ]
            }
          }
        }
      }
    }'
  ```

  ```ruby Ruby highlight={8-18} theme={null}
  response = courier.send_.message(
    message: {
      template: "nt_01kx4h2jdafq8bk9aftxak4b40",
      to: {
        email: "sarah@acme-corp.com"
      },
      providers: {
        resend: {
          override: {
            attachments: [
              {
                filename: "billing.pdf",
                contentType: "application/pdf",
                data: "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh"
              }
            ]
          }
        }
      }
    }
  )
  ```

  ```go Go highlight={10-20} theme={null}
  response, err := client.Send.Message(context.TODO(), courier.SendMessageParams{
  	Message: courier.SendMessageParamsMessage{
  		To: courier.SendMessageParamsMessageToUnion{
  			OfUserRecipient: &shared.UserRecipientParam{
  				Email: courier.String("sarah@acme-corp.com"),
  			},
  		},
  		Template: courier.String("nt_01kx4h2jdafq8bk9aftxak4b40"),
  		Providers: shared.MessageProvidersParam{
  			"resend": shared.MessageProvidersTypeParam{
  				Override: map[string]any{
  					"attachments": []any{
  						map[string]any{
  							"filename": "billing.pdf",
  							"contentType": "application/pdf",
  							"data": "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh",
  						},
  					},
  				},
  			},
  		},
  	},
  })
  ```

  ```java Java highlight={6-14} theme={null}
  SendMessageParams params = SendMessageParams.builder()
      .message(SendMessageParams.Message.builder()
          .to(UserRecipient.builder().email("sarah@acme-corp.com").build())
          .template("nt_01kx4h2jdafq8bk9aftxak4b40")
          .providers(MessageProviders.builder()
              .putAdditionalProperty("resend", JsonValue.from(java.util.Map.of("override", java.util.Map.of(
                      "attachments", java.util.List.of(
                          java.util.Map.of(
                              "filename", "billing.pdf",
                              "contentType", "application/pdf",
                              "data", "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh"
                          )
                      )
                  ))))
              .build())
          .build())
      .build();
  SendMessageResponse response = client.send().message(params);
  ```

  ```php PHP highlight={8-18} theme={null}
  $response = $client->send->message(
    message: [
      'template' => 'nt_01kx4h2jdafq8bk9aftxak4b40',
      'to' => [
        'email' => 'sarah@acme-corp.com',
      ],
      'providers' => [
        'resend' => [
          'override' => [
            'attachments' => [
              [
                'filename' => 'billing.pdf',
                'contentType' => 'application/pdf',
                'data' => 'Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh',
              ],
            ],
          ],
        ],
      ],
    ],
  );
  ```

  ```csharp C# highlight={9-27} theme={null}
  SendMessageParams parameters = new()
  {
      Message = new()
      {
          To = new UserRecipient { Email = "sarah@acme-corp.com" },
          Template = "nt_01kx4h2jdafq8bk9aftxak4b40",
          Providers = new Dictionary<string, MessageProvidersType>()
          {
              {
                  "resend",
                  new()
                  {
                      Override = JsonSerializer.Deserialize<Dictionary<string, JsonElement>>(
                          """
                          {
                            "attachments": [
                              {
                                "filename": "billing.pdf",
                                "contentType": "application/pdf",
                                "data": "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh"
                              }
                            ]
                          }
                          """
                      ),
                  }
              },
          },
      },
  };

  var response = await client.Send.Message(parameters);
  ```

  ```bash CLI highlight={5} wrap theme={null}
  courier send message \
    --api-key "$COURIER_API_KEY" \
    --message.to '{"email": "sarah@acme-corp.com"}' \
    --message.template nt_01kx4h2jdafq8bk9aftxak4b40 \
    --message.providers '{"resend": {"override": {"attachments": [{"filename": "billing.pdf", "contentType": "application/pdf", "data": "Q29uZ3JhdHVsYXRpb25zLCB5b3UgY2FuIGJhc2U2NCBkZWNvZGUh"}]}}}'
  ```

  ```text MCP theme={null}
  With Courier MCP, send my template to sarah@acme-corp.com with an attachment through Resend.
  ```
</CodeGroup>

## Provider details

```text theme={null}
resend
```

Courier recommends routing to the channel. Naming this key in `routing.channels` instead is supported, and sends through just this provider.

<Card title="Send to a specific provider" icon="bullseye-arrow" href="/docs/send/send-to-a-provider" horizontal arrow="true">
  When that is worth doing, and what you give up: failover, channel priority, and providers you add later.
</Card>