Java

// src/main/java/com/acme/Email.java
package com.acme;

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.time.Duration;

public final class Email {
    private static final HttpClient CLIENT = HttpClient.newBuilder()
            .connectTimeout(Duration.ofSeconds(10))
            .build();

    /** The key comes from https://emails.sh/dashboard/api-keys, via EMAILSSH_API_KEY. */
    private static final String KEY = System.getenv("EMAILSSH_API_KEY");

    public static String send(String to, String subject, String html) throws Exception {
        String body = """
                {
                  "from": "Acme <hello@acme.com>",
                  "to": ["%s"],
                  "subject": "%s",
                  "html": "%s"
                }
                """.formatted(escape(to), escape(subject), escape(html));

        HttpRequest request = HttpRequest.newBuilder(URI.create("https://emails.sh/v1/emails"))
                .header("Authorization", "Bearer " + KEY)
                .header("Content-Type", "application/json")
                .timeout(Duration.ofSeconds(10))
                .POST(HttpRequest.BodyPublishers.ofString(body))
                .build();

        HttpResponse<String> response = CLIENT.send(request, HttpResponse.BodyHandlers.ofString());
        if (response.statusCode() >= 400) {
            // The body carries an error code and a message saying what to do
            // about it, so keep the whole thing in the exception.
            throw new IllegalStateException("emails.sh refused the send: " + response.body());
        }
        return response.body();
    }

    private static String escape(String value) {
        return value.replace("\\", "\\\\").replace("\"", "\\\"");
    }

    public static void main(String[] args) throws Exception {
        System.out.println(send("ada@example.com", "Your receipt from Acme", "<p>Thanks for your order.</p>"));
    }
}

import com.fasterxml.jackson.databind.ObjectMapper;
import java.util.List;
import java.util.Map;

private static final ObjectMapper MAPPER = new ObjectMapper();

String body = MAPPER.writeValueAsString(Map.of(
        "from", "Acme <hello@acme.com>",
        "to", List.of(to),
        "subject", subject,
        "html", html,
        "idempotency_key", "receipt-" + orderId));