Send Email with Java

REST API example

Java 11 and newer include java.net.http.HttpClient.

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

var json = """
{
  "from": "Acme <alerts@example.com>",
  "to": ["user@example.com"],
  "subject": "Welcome",
  "html_body": "<p>Hello from Acme</p>",
  "text_body": "Hello from Acme",
  "tags": ["onboarding"]
}
""";

var request = HttpRequest.newBuilder()
    .uri(URI.create("https://api.postscale.io/v1/send"))
    .header("Authorization", "Bearer " + System.getenv("POSTSCALE_API_KEY"))
    .header("Content-Type", "application/json")
    .POST(HttpRequest.BodyPublishers.ofString(json))
    .build();

var response = HttpClient.newHttpClient()
    .send(request, HttpResponse.BodyHandlers.ofString());

if (response.statusCode() >= 400) {
    throw new IllegalStateException("Email send failed: " + response.body());
}

In production, build JSON with your normal serializer rather than string concatenation.

SMTP relay with Jakarta Mail

Use SMTP relay for older applications and frameworks that already use mail sessions.

mail.smtp.host=smtp.postscale.io
mail.smtp.port=587
mail.smtp.auth=true
mail.smtp.starttls.enable=true

Store SMTP credentials in secrets management. Do not reuse a dashboard password.

Service design

Create one EmailSender interface:

public interface EmailSender {
    void sendTransactional(EmailMessage message);
}

Your controllers and jobs should not know whether the current implementation uses Postscale, SendGrid, Mailgun, SES, SMTP, or a test fake.

Retry and idempotency

1. Retry temporary transport failures with backoff.
2. Do not retry validation errors.
3. Store an application event ID for each send.
4. Process webhooks idempotently by event ID.
5. Keep final delivery state separate from initial accepted state.

Read SMTP errors and retries before moving high-volume Java services.

References

• Java HTTP Client API
• Jakarta Mail project
• Postscale SMTP relay guide
• Webhook retries and idempotency