smtp4dev - the mail server for development
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smtp4dev - the mail server for development

A dummy SMTP server for Windows, Linux, Mac OS-X (and maybe elsewhere where .NET Core is available). This repository hosts the development of v3 which will have a re-written web UI so that it can be shared amongst members of a team (the most requested feature in v2). The version hosted on this repo is in heavy development. Grab the [stable(r) v2 version] (a Windows only desktop app) ( if you want something feature complete which you can just double click on and use.

If you find smtp4dev useful, please consider supporting further development by making a donation:


Build status Github Releases (+270k when prev hosted on Codeplex)


Screenshot 1 Screenshot 2

How to run smtp4dev by downloading from github releases

The MacOS release is totally untested. Please contribute instructions on how to use and feedback on any issues

  • Download a release and unzip.

  • On Linux chmod +x the Rnwood.Smtp4dev file to make it executable

  • Edit appsettings.json and set the port number you want the SMTP server to listen on.

  • Run Rnwood.Smtp4dev (.exe on Windows). (If you downloaded the noruntime version, you need the .NET Core 2.0 runtime on your machine and you should execute dotnet Rnwood.Smtpdev.dll to run it.)

  • Open your browser at http://localhost:5000 (to run the web server on a different port or make it listen on interfaces other than loopback, add the command line arg --server.urls "" when starting the executable.

  • Now configure your apps which send mail, to use the SMTP server on the machine where SMTP4dev is running (localhost if they are on the same machine), and using the port you selected (25 by default).

How to run smtp4dev as a dotnet global tool

If you're using the .NET Core SDK 2.1 or greater, you can install smtp4dev as a global tool using the following command:

dotnet tool install -g Rnwood.Smtp4dev --version "3.0.0-*"

Then you can start smtp4dev by running


How to run smtp4dev in Docker

Docker images for both Windows and Linux are available. To run with the web interface on port 3000 and SMTP on port 2525:


docker run -p 3000:80 -p 2525:25 rnwood/smtp4dev:linux-amd64-v3


docker run -p 3000:80 -p 2525:25 rnwood/smtp4dev:windows-amd64-v3

Sorry no unified cross platform image tag available yet. To see the full list of available tags see the Docker hub page for smtp4dev.

How to run smtp4dev as a service (Windows only)

A service in Windows can be installed using New-Service in PowerShell, or sc in both command line or PowerShell. If you use sc in PowerShell, it must be run as sc.exe. sc is an alias for Set-Content.

Install service in PowerShell

New-Service -Name Smtp4dev -BinaryPathName "{PathToExe} --service"

Install service in Cmd or PowerShell

sc.exe create Smtp4dev binPath= "{PathToExe} --service"


Changing how many messages or sessions are kept

smtp4dev keeps the latest 100 messages and 100 sessions by default. The --messagestokeep X and --sessionstokeep X command line options can override this.

Changing where the database file is saved

By default smtp4dev will create a Sqlite DB named database.db in application root direction when it runs.

To change the path of this file use the --db "/path/to/file.db" command line option or edit ServerOptions\Database in appsettings.json

To use an in memory DB use an empty string (e.g. --db="")). All session and messages will be lost when the process exits. (Docker: The = in the example is important as Docker will eat the empty quotes if ommitted.)

Changing the SMTP port

smtp4dev listens on (all interfaces) port 25 by default. To change this either edit ServerOptions\Port in the appsettings.json before startup or add the --smtpport command line options (e.g. --smtpport 2525).