Add README.md for SMTP2Graph project

Added detailed information about SMTP2Graph, including features, requirements, environment variables, usage instructions, security considerations, and operational workflow.

Author: krisdb2009

README.md

@@ -0,0 +1,51 @@
+# SMTP2Graph
+
+SMTP2Graph is a lightweight SMTP listener that accepts incoming email messages over TCP (port 25) and forwards them to Microsoft 365 mailboxes using the Microsoft Graph API. This allows you to relay SMTP traffic into Exchange Online without exposing traditional SMTP relay services.
+
+## Features
+- Minimal SMTP command support (HELO/EHLO, MAIL FROM, RCPT TO, DATA, QUIT)
+- Converts incoming MIME messages to Base64 and sends via Microsoft Graph `/users/{id}/sendMail`
+- Runs as a standalone .NET application or inside Docker
+
+## Requirements
+- .NET 9.0 SDK for building
+- Microsoft Graph API credentials (App registration with `Mail.Send` permission)
+
+## Environment Variables
+| Variable       | Description                                      |
+|---------------|--------------------------------------------------|
+| `TENANT_ID`   | Azure AD tenant ID                               |
+| `CLIENT_ID`   | App registration client ID                       |
+| `CLIENT_SECRET`| App registration client secret                  |
+
+## Usage
+### Build and Run Locally
+```bash
+dotnet restore
+dotnet publish SMTP2Graph -c Release -o ./publish
+cd publish
+TENANT_ID="<tenant>" CLIENT_ID="<client>" CLIENT_SECRET="<secret>" ./SMTP2Graph
+```
+
+### Docker Pull and Run
+```bash
+# Pull the latest image
+docker pull ghcr.io/belowaverage-org/smtp2graph:latest
+
+# Run the container
+docker run -d \
+  -e TENANT_ID="<tenant>" \
+  -e CLIENT_ID="<client>" \
+  -e CLIENT_SECRET="<secret>" \
+  -p 25:25 ghcr.io/belowaverage-org/smtp2graph:latest
+```
+
+## Security Considerations
+- **Do not expose this container publicly without authentication and TLS.**
+- Implement IP allowlists or run behind a secure proxy.
+- Use Application Access Policies in Microsoft 365 to restrict which mailboxes the app can send as.
+
+## How It Works
+1. Listens on port 25 for SMTP commands.
+2. Reads MIME content after `DATA` command.
+3. Encodes MIME as Base64 and sends via Microsoft Graph using `sendMail` endpoint.