Why SMTP Relay Matters for Email Signature Management
If you manage email infrastructure for a company or as a hosting provider, you know the challenge: ensuring every outgoing email carries a consistent, professional signature — regardless of which device or email client the sender uses. The most reliable way to achieve this is through server-side email signature injection via SMTP relay.
Unlike client-side solutions that depend on Outlook plugins or manual user configuration, an SMTP relay approach intercepts outgoing emails at the transport level and applies signatures automatically. This guarantees 100% compliance, works across all devices (including mobile), and eliminates the risk of employees forgetting to update their signature or sending emails without one.
In this tutorial, we walk you through the fundamentals of configuring SMTP relay for signature injection, covering both Microsoft 365 environments and generic SMTP servers such as cPanel, Plesk, Roundcube, and DirectAdmin.
What Is SMTP Relay in the Context of Email Signatures?
SMTP relay is a process where outgoing email is routed through an intermediate server before reaching the recipient. In the context of email signature management, this intermediate server inspects the outgoing message, identifies the sender, and injects the appropriate HTML signature before forwarding the email to its final destination.
This method is sometimes referred to as transport-level signature injection or server-side signature stamping. It is the gold standard for organisations that want centralised control over their email branding.
Option 1: Configuring SMTP Relay in Microsoft 365
Microsoft 365 offers built-in mail flow rules (transport rules) and connectors that make it possible to route outgoing mail through an external signature service. Here is how to set it up:
Step 1: Create an Outbound Connector
In the Microsoft 365 Exchange Admin Center, navigate to Mail flow → Connectors. Create a new connector with the following settings:
- From: Office 365
- To: Partner organisation (your signature injection service)
- Set the smart host to the SMTP relay address provided by your signature management platform (e.g., Badex Signature provides a dedicated relay endpoint for each tenant).
- Enable TLS encryption to ensure emails are transmitted securely.
Step 2: Create a Mail Flow Rule
Navigate to Mail flow → Rules and create a new rule:
- Apply the rule to all outgoing messages (or scope it to specific groups/departments).
- Set the action to redirect the message to the connector you created in Step 1.
- Add exceptions if needed — for example, exclude encrypted or digitally signed messages.
Step 3: Verify DNS and SPF Records
To ensure deliverability, update your SPF record to include the relay server's IP addresses or domain. This prevents recipient servers from flagging your relayed emails as spam. Your signature management provider should supply the exact SPF include statement to add.
Step 4: Test Thoroughly
Send test emails from multiple clients — Outlook desktop, Outlook mobile, webmail — and verify that signatures are being injected correctly. Check that formatting, images, and links render properly across different email clients.
Option 2: Configuring SMTP Relay on cPanel, Plesk, and Other Hosting Panels
For organisations and hosting providers not using Microsoft 365, SMTP relay can be configured at the mail server level. This is particularly relevant for European hosting companies managing multiple client domains on cPanel, Plesk, DirectAdmin, or custom Postfix/Exim setups.
cPanel (Exim-based)
- Access WHM and navigate to Exim Configuration Manager.
- In the Section: Router Configuration, add a custom router that redirects outgoing mail to the signature relay smart host.
- Configure authentication credentials if required by the relay service.
- Ensure the relay is set to process the email and return it for final delivery, or forward it directly to the recipient's mail server.
Plesk (Postfix-based)
- Access the Plesk server via SSH.
- Edit the Postfix configuration (
main.cf) to add a relayhost directive pointing to your signature injection relay. - Configure SASL authentication if the relay requires credentials.
- Reload Postfix to apply changes.
DirectAdmin and Roundcube
- DirectAdmin servers typically use Exim, so the configuration is similar to cPanel. Edit the Exim configuration to add a smart host route for outgoing mail.
- Roundcube is a webmail client, not a mail server — so the relay must be configured at the underlying MTA (Postfix, Exim, or Dovecot proxy) rather than in Roundcube itself.
Key Considerations for a Successful Setup
Regardless of your mail platform, keep these best practices in mind when configuring SMTP relay for signature injection:
- TLS encryption is non-negotiable. Especially under GDPR and European data protection regulations, all email data in transit must be encrypted. Ensure your relay connection uses TLS 1.2 or higher.
- SPF, DKIM, and DMARC alignment. Routing email through a relay can break authentication if not configured properly. Make sure your SPF record includes the relay, DKIM signing is preserved or re-applied, and DMARC policies are aligned.
- Minimise latency. Choose a relay provider with servers in Europe (or your region) to avoid adding noticeable delay to email delivery. Badex Signature, for example, operates relay infrastructure optimised for European businesses.
- Centralised template management. The whole point of server-side injection is centralised control. Use a platform that lets you design templates, assign them by department or user, and update signatures across the entire organisation with one click.
- Test across clients. HTML email rendering varies wildly between Outlook, Apple Mail, Gmail, and mobile clients. Always test your signature templates in multiple environments before deploying to production.
How Badex Signature Simplifies the Process
At Badex Signature, we built our platform specifically to make SMTP relay-based signature injection easy — whether you run Microsoft 365 or a traditional SMTP mail server on cPanel, Plesk, DirectAdmin, or any other hosting environment.
Here is what sets us apart:
- Dual compatibility: Full support for both Microsoft 365 (via connectors and mail flow rules) and generic SMTP relay for any mail server.
- European-hosted infrastructure: Our relay servers are hosted in Europe, ensuring GDPR compliance and low-latency processing.
- No client-side installation: Signatures are applied at the server level, so there is nothing to install on user devices.
- Dynamic fields: Automatically populate signatures with user data such as name, job title, phone number, and department — pulled from Active Directory, Microsoft 365, or a custom CSV import.
- Campaign banners: Add promotional banners or legal disclaimers to signatures and manage them centrally.
Getting Started
Configuring SMTP relay for email signature injection may seem complex at first, but with the right tools and a clear step-by-step approach, it is a straightforward process that delivers immediate results in brand consistency and professional communication.
If you are an IT administrator or hosting provider looking for a reliable, European-hosted signature management solution that works with your existing infrastructure, Badex Signature is ready to help. Contact us for a personalised walkthrough or start your free trial today.