> ## 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`, the default import of the v7 Node SDK. 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 MailerSend through Courier

> Connect MailerSend with an API token and from address, then override 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

* [A MailerSend account](https://www.mailersend.com/)
* A verified sending domain in MailerSend

## Setup

<Steps>
  <Step title="Verify your domain">
    In MailerSend, open [Domains](https://app.mailersend.com/domains), add your domain, and complete DNS verification.

    <Frame>
      <img src="https://mintcdn.com/courier-4f1f25dc/xDJPBS8EQ58X_0-v/assets/mailersend-domains.webp?fit=max&auto=format&n=xDJPBS8EQ58X_0-v&q=85&s=64779cd2f355b73e5acc196bb87669ff" width="1244" height="390" data-path="assets/mailersend-domains.webp" />
    </Frame>
  </Step>

  <Step title="Generate an API token">
    Click "Manage" on your domain and [generate an API token](https://www.mailersend.com/help/managing-api-tokens).

    <Frame>
      <img src="https://mintcdn.com/courier-4f1f25dc/xDJPBS8EQ58X_0-v/assets/mailersend-api-tokens.webp?fit=max&auto=format&n=xDJPBS8EQ58X_0-v&q=85&s=f4d58c3b1dd5bf2583deab92ada71d20" width="1242" height="270" data-path="assets/mailersend-api-tokens.webp" />
    </Frame>
  </Step>

  <Step title="Configure in Courier">
    In Courier, open the <AppLink href="https://app.courier.com/integrations/catalog/mailersend">MailerSend Integration</AppLink> page. Enter your API token and From Address, then click "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.

You can override any field MailerSend's [/v1/email endpoint](https://developers.mailersend.com/api/v1/email.html#send-an-email) supports. Use an override when Courier does not support a field yet, or to replace a value Courier generates.

This example swaps the API key and the sender in `config`:

<CodeGroup>
  ```javascript Node.js highlight={8-15} theme={null}
  const { requestId } = await courier.send.message({
    message: {
      template: "nt_01kx4h2jdafq8bk9aftxak4b40",
      to: {
        email: "sarah@acme-corp.com",
      },
      providers: {
        mailersend: {
          override: {
            config: {
              apiKey: "<your override API key>",
              fromAddress: "alternate-sender@yourdomain.com",
            },
          },
        },
      },
    },
  });
  ```

  ```python Python highlight={8-15} theme={null}
  response = client.send.message(
      message={
          "template": "nt_01kx4h2jdafq8bk9aftxak4b40",
          "to": {
              "email": "sarah@acme-corp.com",
          },
          "providers": {
              "mailersend": {
                  "override": {
                      "config": {
                          "apiKey": "<your override API key>",
                          "fromAddress": "alternate-sender@yourdomain.com",
                      },
                  },
              },
          },
      },
  )
  ```

  ```bash cURL highlight={11-18} 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": {
          "mailersend": {
            "override": {
              "config": {
                "apiKey": "<your override API key>",
                "fromAddress": "alternate-sender@yourdomain.com"
              }
            }
          }
        }
      }
    }'
  ```

  ```ruby Ruby highlight={8-15} theme={null}
  response = courier.send_.message(
    message: {
      template: "nt_01kx4h2jdafq8bk9aftxak4b40",
      to: {
        email: "sarah@acme-corp.com"
      },
      providers: {
        mailersend: {
          override: {
            config: {
              apiKey: "<your override API key>",
              fromAddress: "alternate-sender@yourdomain.com"
            }
          }
        }
      }
    }
  )
  ```

  ```go Go highlight={10-17} 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{
  			"mailersend": shared.MessageProvidersTypeParam{
  				Override: map[string]any{
  					"config": map[string]any{
  						"apiKey": "<your override API key>",
  						"fromAddress": "alternate-sender@yourdomain.com",
  					},
  				},
  			},
  		},
  	},
  })
  ```

  ```java Java highlight={6-11} 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("mailersend", JsonValue.from(java.util.Map.of("override", java.util.Map.of(
                      "config", java.util.Map.of(
                          "apiKey", "<your override API key>",
                          "fromAddress", "alternate-sender@yourdomain.com"
                      )
                  ))))
              .build())
          .build())
      .build();
  SendMessageResponse response = client.send().message(params);
  ```

  ```php PHP highlight={8-15} theme={null}
  $response = $client->send->message(
    message: [
      'template' => 'nt_01kx4h2jdafq8bk9aftxak4b40',
      'to' => [
        'email' => 'sarah@acme-corp.com',
      ],
      'providers' => [
        'mailersend' => [
          'override' => [
            'config' => [
              'apiKey' => '<your override API key>',
              'fromAddress' => 'alternate-sender@yourdomain.com',
            ],
          ],
        ],
      ],
    ],
  );
  ```

  ```csharp C# highlight={9-24} theme={null}
  SendMessageParams parameters = new()
  {
      Message = new()
      {
          To = new UserRecipient { Email = "sarah@acme-corp.com" },
          Template = "nt_01kx4h2jdafq8bk9aftxak4b40",
          Providers = new Dictionary<string, MessageProvidersType>()
          {
              {
                  "mailersend",
                  new()
                  {
                      Override = JsonSerializer.Deserialize<Dictionary<string, JsonElement>>(
                          """
                          {
                            "config": {
                              "apiKey": "<your override API key>",
                              "fromAddress": "alternate-sender@yourdomain.com"
                            }
                          }
                          """
                      ),
                  }
              },
          },
      },
  };

  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 '{"mailersend": {"override": {"config": {"apiKey": "<your override API key>", "fromAddress": "alternate-sender@yourdomain.com"}}}}'
  ```

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

### Attachments

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: {
        mailersend: {
          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": {
              "mailersend": {
                  "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": {
          "mailersend": {
            "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: {
        mailersend: {
          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{
  			"mailersend": 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("mailersend", 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' => [
        'mailersend' => [
          '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>()
          {
              {
                  "mailersend",
                  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 '{"mailersend": {"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 MailerSend.
  ```
</CodeGroup>

## Troubleshooting

<Accordion title="MailerSend 422 response code">
  A [422 error](https://www.mailersend.com/help/how-to-start-sending-emails#rest-api) from MailerSend has several possible causes:

  | Error                                 | Cause                                                                                                                    |
  | ------------------------------------- | ------------------------------------------------------------------------------------------------------------------------ |
  | From email must be verified           | The domain of the from email address must match the domain that the API token is from.                                   |
  | Must provide HTML/text or template ID | The API request is missing content; provide either an HTML/text body or a template ID.                                   |
  | File type not supported               | The attachment is not a [supported file type](https://developers.mailersend.com/api/v1/email.html#supported-file-types). |
  | Reply-to must be a valid email        | The `reply_to` parameter is not a valid email address.                                                                   |
  | Email quota reached                   | The account's quota has been reached. Ensure your account is approved for production sending.                            |

  The most common cause is an unverified sending domain. [Verify your domain with MailerSend](https://www.mailersend.com/help/how-to-verify-and-authenticate-a-sending-domain) before using it with Courier.
</Accordion>

## Provider details

```text theme={null}
mailersend
```

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>