This is a simple SMTP simulator server for bounces and complaints.

For Hyvor Relay customers, the email domain is simulator.relay.hyvor.com. For example, to simulate a busy mailbox, send an email to busy@simulator.relay.hyvor.com.

Installation

Using Docker

hyvor/smtp-simulator is a minimal Docker image that exposes port 25 for SMTP.

docker run -p 25:25 hyvor/smtp-simulator

Using Docker Compose

services:
    smtp-simulator:
        image: hyvor/smtp-simulator
        ports:
        - "25:25"

From Source

git clone https://github.com/hyvor/smtp-simulator
cd smtp-simulator
go build -o smtp-simulator .
./smtp-simulator

Environment Variables

• DOMAIN (default: localhost): The domain name of the SMTP server. You should set up a MX record for this domain to point to the server's IP address. This is used in the HELO/EHLO command and in the From address of bounce emails.

Email Addresses

Send emails to the following addresses to simulate different scenarios.

Synchronous Responses

These emails respond with a bounce immediately within the SMTP transaction after the DATA command is completed.

Email Local Part	Description	Status Code	Enhanced Code
accept@	Accepts the email and simulates a successful delivery.	250	2.0.0
busy@	Simulates a busy mailbox.	450	4.2.1
tempfail@	Simulates a temporary failure.	451	4.3.0
missing@	Simulates a hard bounce.	550	5.1.1
disabled@	Simulates a disabled email address.	550	5.1.2
spam@	Simulates a spam rejection.	550	5.7.1

Asynchronous Bounces

These emails accept the message initially but later send a bounce notification (DSN) back to the sender. The bounce is sent as per RFC3464.

Email Local Part	Description	Status Code	Enhanced Code
missing+async@	Simulates a hard bounce.	550	5.1.1
disabled+async@	Simulates a disabled email address.	550	5.1.2
spam+async@	Simulates a spam rejection.	550	5.7.1

• If there are multiple recipients with different bounce types, a single DSN will be sent with all the failed recipients listed.

Complaints

These emails accept the message initially but later send a complaint notification back to the sender. The complaint is sent as per RFC5965.

Email Local Part	Description
complaint@	Simulates a user complaint.