Nager.MtaMilter is a C# implementation of an MTA Milter, providing the core logic for the Milter protocol. This project enables seamless integration with popular mail servers such as Postfix, Sendgrid, and Stalwart. It focuses primarily on implementing the Milter protocol and handling various Milter events.
- Pure C# implementation of the Milter protocol
- Compatible with Postfix, Sendgrid, Stalwart, and other MTA systems
- Event-driven handling of mail processing steps
- Lightweight and modular design
Nager.MtaMilter can react to the following Milter protocol events:
| Event | Command Byte | Description | Required |
|---|---|---|---|
SMFIC_OPTNEG |
0x4F |
Protocol negotiation | Yes |
SMFIC_QUIT |
0x51 |
Quit communication | Yes |
SMFIC_MACRO |
0x44 |
Macro definition | No |
SMFIC_CONNECT |
0x43 |
Connection information | No |
SMFIC_MAIL |
0x4D |
MAIL FROM command | No |
SMFIC_RCPT |
0x52 |
RCPT TO command | No |
SMFIC_DATA |
0x54 |
DATA command, signals the start of the message body but contains no actual data | No |
SMFIC_HEADER |
0x4C |
Mail header | No |
SMFIC_EOH |
0x4E |
End of headers | No |
SMFIC_BODY |
0x42 |
Message body | No |
SMFIC_BODYEOB |
0x45 |
End of body | No |
The diagram illustrates a typical communication sequence between the mail server and the Milter server.
The communication always starts with SMFIC_OPTNEG, which negotiates protocol options. It always ends with SMFIC_QUIT, indicating the termination of the session. Between these two commands, various other Milter messages may be exchanged, depending on the email transaction.
graph TD;
SMFIC_OPTNEG-->SMFIC_______1;
SMFIC_______1-->SMFIC_______2;
SMFIC_______2-->SMFIC_QUIT;
Tested with Stalwart Mail Server. During development, I used the Stalwart Mail Server to test and validate Nager.MtaMilter. The integration was successful, ensuring compatibility with the Milter protocol.
Find installation and configuration details in the official documentation: Stalwart Mail Server Docs
docker run -d -ti -p 443:443 -p 8080:8080 -p 25:25 -p 587:587 -p 465:465 -p 143:143 -p 993:993 -p 4190:4190 -p 110:110 -p 995:995 -v c:\Temp\stalwart:/opt/stalwart-mail --name stalwart-mail stalwartlabs/mail-server:latest
Send test mail via telnet
helo test.de
mail from:<[email protected]>
rcpt to:<[email protected]>
data
354 Start mail input; end with <CRLF>.<CRLF>
Date: Sun, 09 Mar 2025 06:28:30 -0700
From: Max <[email protected]>
To: "Muster" <[email protected]>
Subject: Test Mail
test mail
.
Milter Configuration
Contributions are welcome! Feel free to submit issues, feature requests, or pull requests via GitHub.
This project is licensed under the MIT License.
Created and maintained by nager.