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\",\n },\n priority: \"high\",\n});\n~~~~\n\nSince Upyo 0.6.0, internationalized mailbox addresses are accepted, including\nUTF-8 local parts and Unicode domains:\n\n~~~~ typescript\nconst message = createMessage({\n from: \"josé@example.com\",\n to: \"用户@例子.广告\",\n subject: \"Hello\",\n content: { text: \"Welcome!\" },\n});\n~~~~\n\nSupport for delivering these addresses depends on the selected transport. The\n*@upyo/smtp* transport negotiates RFC 6531 SMTPUTF8 automatically.\n\n### Adding attachments\n\nAttachments can be added using the standard [`File`] API:\n\n~~~~ typescript\nimport { createMessage } from \"@upyo/core\";\n\nconst message = createMessage({\n from: \"sender@example.com\",\n to: \"recipient@example.net\",\n subject: \"Document attached\",\n content: { text: \"Please find the document attached.\" },\n attachments: [\n new File(\n [await fetch(\"document.pdf\").then(r => r.arrayBuffer())],\n \"document.pdf\",\n { type: \"application/pdf\" }\n ),\n ],\n});\n~~~~\n\n[`File`]: https://developer.mozilla.org/en-US/docs/Web/API/File\n\n### Handling receipts\n\nAll transport operations return `Receipt` objects that use discriminated unions\nfor type-safe error handling:\n\n~~~~ typescript\nimport type { Receipt } from \"@upyo/core\";\n\nfunction handleReceipt(receipt: Receipt) {\n if (receipt.successful) {\n console.log(\"Message sent with ID:\", receipt.messageId);\n } else {\n console.error(\"Send failed:\", receipt.errorMessages.join(\", \"));\n console.error(\"Retryable:\", receipt.retryable ?? false);\n\n for (const error of receipt.errors ?? []) {\n console.error(error.category, error.code, error.provider);\n }\n }\n}\n~~~~\n\nFailed receipts keep the legacy `errorMessages` array and can also carry\nstructured `errors` for programmatic handling. Transports use categories such\nas `auth`, `rate-limit`, `network`, `timeout`, `validation`, `rejected`,\n`server-error`, `service-unavailable`, `configuration`, and `unknown`.\n\nWhen implementing a transport, use `createFailedReceipt()` to keep these fields\nconsistent:\n\n~~~~ typescript\nimport { createFailedReceipt } from \"@upyo/core\";\n\nconst receipt = createFailedReceipt(\"HTTP 429: Too Many Requests\", {\n provider: \"example\",\n statusCode: 429,\n retryAfterMilliseconds: 30_000,\n});\n~~~~\n\n\nImplementing custom transports\n------------------------------\n\nThe `Transport` interface defines the contract for all email providers:\n\n~~~~ typescript\nimport type { Message, Receipt, Transport, TransportOptions } from \"@upyo/core\";\n\nclass MyCustomTransport implements Transport<\"example\"> {\n readonly id = \"example\";\n\n async send(\n message: Message,\n options?: TransportOptions,\n ): Promise