Send email in Kotlin via HTTP API (no SMTP)

Send email from Kotlin with one HTTP call — no JavaMail, no SMTP. The send code with java.net.http, how to receive the reply, and error handling. Free to start.

Sending email from Kotlin usually pulls in JavaMail and SMTP configuration. A free HTTP email API replaces that with the JDK’s built-in HttpClient — and it can read the reply, which an SMTP sender can’t.

Send email in Kotlin

java.net.http.HttpClient works directly from Kotlin, no transport dependency:

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

val client = HttpClient.newHttpClient()
val base = "https://login.ollastack.com"
val token = "fmd_…"

fun post(path: String, json: String): String {
    val req = HttpRequest.newBuilder(URI.create(base + path))
        .header("Authorization", "Bearer $token")
        .header("Content-Type", "application/json")
        .POST(HttpRequest.BodyPublishers.ofString(json))
        .build()
    val res = client.send(req, HttpResponse.BodyHandlers.ofString())
    check(res.statusCode() < 400) { "Request failed: ${res.body()}" }
    return res.body()
}

// create a mailbox once
val mbx = post("/api/mailboxes",
    """{"name":"kotlin","mode":"agent","handle":"kt"}""")
val id = Regex("\"id\":\"(.*?)\"").find(mbx)!!.groupValues[1]

// send
post("/api/mailboxes/$id/send",
    """{"to":"user@example.com","subject":"Hi","text":"Sent from Kotlin."}""")

No SMTP host, no JavaMail session — one HTTPS request returning a msg_… id. (Kotlin’s raw triple-quoted strings make the JSON bodies readable.)

Receive the reply

A GET to the wait endpoint long-polls for the next inbound message:

val req = HttpRequest.newBuilder(
        URI.create("$base/api/mailboxes/$id/wait?timeout=60"))
    .header("Authorization", "Bearer $token").GET().build()
val msg = client.send(req, HttpResponse.BodyHandlers.ofString()).body()
// parse codes[0] for an OTP — extracted for you

That’s the half an SMTP sender can’t do — read inbound mail and pull the code.

Send HTML, not just text

Swap text for html (or send both — text is the fallback for clients that don’t render HTML):

post("/api/mailboxes/$id/send", """
  {"to":"user@example.com","subject":"Welcome",
   "html":"<h1>Welcome</h1><p>Thanks for signing up.</p>",
   "text":"Welcome — thanks for signing up."}
""".trimIndent())

The same endpoint takes both; add a reply_to field to route replies to a specific address.

Why an API beats JavaMail / SMTP

JavaMail makes you configure SMTP and only sends. The built-in HttpClient call sends from an authenticated domain, returns a structured error, and also receives. On Android, run it on Dispatchers.IO and keep the token off-device.

Error handling

The check(...) raises on any >= 400; on 429 back off and retry, reading the JSON error body for the code and message.

See the email API overview, the Java version, and the C# / .NET version.

Get a free token — send and receive from Kotlin, no card.

Last updated June 21, 2026. Spotted something out of date? Email hello@ollastack.com.