fix: Support Exchange servers without authentication

- Make email authentication optional in transporter config
- Only add auth object if both username and password are provided
- Update initialization to only require SMTP_HOST
- Add logging to show whether auth is enabled
- Update documentation with no-auth configuration examples
- Fix "Unrecognized authentication type" error for open relays

Exchange servers configured as internal relays often don't
require authentication. This fix allows using them by leaving
SMTP_USER and SMTP_PASS empty in the .env file.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

EMAIL_CONFIGURATION.md

@@ -8,6 +8,7 @@ MinIO WebUI supports email notifications for storage reports. This guide explain
 
 Configure these variables in your `.env` file:
 
+### With Authentication (Most servers)
 ```bash
 # SMTP Server Configuration
 SMTP_HOST=your-exchange-server.com
@@ -24,6 +25,24 @@ REPORT_RECIPIENT=admin@yourdomain.com
 REPORT_SCHEDULE=0 0 * * 1  # Every Monday at midnight
 ```
 
+### Without Authentication (Open Relay/Internal Exchange)
+```bash
+# SMTP Server Configuration
+SMTP_HOST=exchange.internal.company.com
+SMTP_PORT=25
+SMTP_SECURE=false
+# Leave SMTP_USER and SMTP_PASS empty or don't include them
+SMTP_USER=
+SMTP_PASS=
+
+# Email Settings
+REPORT_SENDER=minio-reports@yourdomain.com
+REPORT_RECIPIENT=admin@yourdomain.com
+
+# Report Schedule (cron format)
+REPORT_SCHEDULE=0 0 * * 1  # Every Monday at midnight
+```
+
 ## Port and Security Settings
 
 ### Port 25 (No TLS/SSL)
@@ -70,11 +89,13 @@ SMTP_SECURE=true
 
 ### Authentication Failed
 
-**Error**: `Invalid login`
+**Error**: `Invalid login` or `Unrecognized authentication type`
 
 **Solution**:
-1. Verify SMTP_USER and SMTP_PASS are correct
-2. For Exchange, use either:
+1. If your Exchange server doesn't require authentication (internal relay):
+   - Leave `SMTP_USER` and `SMTP_PASS` empty in your .env file
+   - Or remove these variables entirely
+2. If authentication is required, verify credentials:
    - Username only: `username`
    - Domain\Username: `DOMAIN\\username`
    - Email format: `username@domain.com`