SMTP Proxy

The SMTP Proxy solves a common problem: many legacy applications can only send via SMTP, but modern email services like Microsoft 365 or Google Gmail require OAuth-based APIs.

The SMTP Proxy listens locally on a configurable port (default: 2525) and accepts standard SMTP connections. The emails are then delivered via the configured provider — either Microsoft Graph API, Google Gmail API or a classic SMTP server.

How It Works

┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│ Legacy App      │────▶│ SMTP Proxy      │────▶│ Email Provider  │
│ (send SMTP)     │     │ (Port 2525)     │     │ • M365 Graph    │
│                 │     │ • Check auth    │     │ • Google Gmail  │
│ Scanner         │     │ • Parse mail    │     │ • SMTP Relay    │
│ ERP System      │     │ • Forward       │     │                 │
│ Monitoring      │     │ • Log           │     │                 │
└─────────────────┘     └─────────────────┘     └─────────────────┘

• Multi-provider — Microsoft 365 (Graph API), Google Gmail (OAuth) or direct SMTP relay.
• SMTP-to-API bridge — Legacy applications send via SMTP, the proxy forwards via modern APIs.
• Attachment support — Multipart messages and attachments are correctly processed and forwarded.
• SMTP authentication — Optional PLAIN/LOGIN authentication for incoming connections.
• Sender validation — For M365: allowed sender list (full emails or domain patterns).
• Secret expiry monitoring — Automatic warning before M365 client secret expiration.
• Delivery log — Every sent and failed email is logged in the database.
• Connection testing — Test provider connectivity before production use.
• Statistics — Uptime, total sent, error rate and last error message.

1. Open component — Navigate to Components > SMTP Proxy in AdminUI.
2. Choose provider — Select the email provider: Microsoft 365, Google Gmail or direct SMTP.
3. Configure provider — Enter provider-specific credentials (e.g. Tenant ID and Client Secret for M365).
4. Set sender — Define the default sender address.
5. Test connection — Send a test email to verify the provider configuration.
6. Set listen address — Default: 127.0.0.1:2525. For network access: 0.0.0.0:2525.
7. Enable auth — For network access: enable SMTP AUTH and set credentials.
8. Start proxy — Enable and start the SMTP proxy.
9. Configure applications — Configure legacy applications to send SMTP via host:2525.

• Prefer M365 — Microsoft 365 offers the best deliverability and requires no open SMTP ports.
• Monitor client secret expiry — For M365, the client secret expiration date is tracked. Renew it in time.
• Enable authentication — Enable SMTP AUTH to prevent unauthorized sending, especially when the proxy is network-accessible.
• Send test mail — Send a test email after every configuration change to verify delivery.
• Maintain sender list — For M365: define allowed sender addresses or domains to prevent abuse.
• Check logs — The SMTP proxy logs every sent/failed mail in the database.

1. Microsoft 365 as Provider

Host: 127.0.0.1
Port: 2525
Provider: m365
Tenant ID: xxxx-xxxx
Client ID: yyyy-yyyy
Client Secret: zzzz
Sender: noreply@company.com

Legacy applications send to 127.0.0.1:2525, the proxy forwards via Microsoft Graph API.

2. SMTP Relay with STARTTLS

Host: 127.0.0.1
Port: 2525
Provider: smtp
SMTP Host: mail.company.com
SMTP Port: 587
STARTTLS: true
Username: smtp_user
Password: ***

3. With Authentication (Network-Safe)

Host: 0.0.0.0  (network access)
Port: 2525
Auth required: true
Auth Username: relay_user
Auth Password: ***
Provider: m365