Ktor + Mailexam

Minimal Ktor example that sends test mail through Mailexam SMTP via Jakarta Mail.

Based on the Mailexam Ktor guide.

What you need

• A Mailexam account and a project with SMTP credentials.
• JDK 17+ and Gradle (wrapper included).

From your Mailexam welcome email or dashboard:

Variable	Description
MAILEXAM_LOGIN	SMTP login (for example, xxxxx)
MAILEXAM_PASSWORD	SMTP password (paired with the login)
Host	{MAILEXAM_LOGIN}.mailexam.ru (built in Mail.kt)

Quick start (host)

1. Export Mailexam credentials (or copy from the example file):

cp .env.example .env
export $(grep -v '^#' .env | xargs)

2. Edit .env:

MAILEXAM_LOGIN=YOUR_LOGIN
MAILEXAM_PASSWORD=YOUR_PASSWORD
MAILEXAM_PORT=587
MAIL_FROM=noreply@example.test

3. Run the server:

./gradlew run

The server listens on http://127.0.0.1:8080 by default.

4. Send a test message:

curl -X POST http://127.0.0.1:8080/mail/test \
  -H 'Content-Type: application/json' \
  -d '{"to":"user@example.test","subject":"Test","body":"Hello"}'

The message appears in the Mailexam dashboard → your project → inbox.

Environment variables

Variable	Required	Default	Description
MAILEXAM_LOGIN	yes	—	SMTP login; also used to build the host name
MAILEXAM_PASSWORD	yes	—	SMTP password
MAILEXAM_PORT	no	587	SMTP port (587, 2525, or 25)
MAIL_FROM	no	noreply@example.test	Sender address (any test address is fine)
HTTP_HOST	no	127.0.0.1	HTTP bind address
HTTP_PORT	no	8080	HTTP listen port

For port 587 and 2525, STARTTLS is enabled (mail.smtp.starttls.enable=true). For port 25, it is disabled.

Project layout

.
├── build.gradle.kts
├── src/main/kotlin/Mail.kt
├── src/main/kotlin/Application.kt   # POST /mail/test
├── .env.example
├── Dockerfile                       # for local debugging only
└── docker-compose.yml

Docker (debugging)

Docker is provided for local debugging. For day-to-day development, run the app on the host with ./gradlew run (see above).

cp .env.example .env
# edit .env with your credentials

docker compose up --build

Then call the same endpoint on the mapped port:

curl -X POST http://127.0.0.1:8080/mail/test \
  -H 'Content-Type: application/json' \
  -d '{"to":"user@example.test","subject":"Test","body":"Hello"}'

Inside the container the server binds to 0.0.0.0:8080.

CI

Set these secrets in your CI environment:

variables:
  MAILEXAM_LOGIN: $MAILEXAM_LOGIN
  MAILEXAM_PASSWORD: $MAILEXAM_PASSWORD
  MAILEXAM_PORT: "587"
  MAIL_FROM: "noreply@example.test"

After sending a message in a test, verify delivery via the Mailexam API.

Troubleshooting

TLS or connection error

• Host must be {login}.mailexam.ru, where {login} matches MAILEXAM_LOGIN.
• Login and password must come from the same Mailexam project.

Event loop blocking

• Transport.send runs inside withContext(Dispatchers.IO).

Message not in the dashboard

• Open the inbox of the same Mailexam project.
• Check Ktor logs when Mail.sendTest throws.

See also

• Mailexam Ktor guide (wiki)
• Ktor documentation
• Mailexam API documentation