Knowledge Base
How to Enable and Configure S/MIME in Synology MailPlus?
How to Enable and Configure S/MIME in Synology MailPlus?
Overview
This guide provides the step-by-step process for enabling and configuring S/MIME email encryption and digital signing in the Synology MailPlus web client, ensuring secure, authenticated, and compliant email communication.
Purpose
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Encrypt outgoing emails to prevent unauthorized access
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Digitally sign emails to verify sender identity
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Improve compliance with security and privacy requirements
Prerequisites
Before you begin, ensure the following:
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You have access to the Synology MailPlus web client
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You have a valid S/MIME certificate (usually a .pfx or .p12 file)
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You know the password for the S/MIME certificate
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Your user account has permission to manage mail encryption settings
Procedure: Enable S/MIME in Synology MailPlus
Step 1: Open MailPlus Settings
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Log in to the Synology MailPlus web client.
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Click the Settings (gear icon) in the top-right corner.
Step 2: Enable Mail Encryption
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Navigate to Settings > Mail Encryption.
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Select Enable encryption.
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From the encryption method drop-down menu, select S/MIME.
Step 3: Configure Default Encryption Options
Choose your default settings:
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Encrypt messages by default — recommended: Off
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Digitally sign messages by default — recommended: On
Note:
You can adjust these options for individual emails while composing a message.
Step 4: Manage Keys and Certificates
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Click Keys & Certificates.
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Select the S/MIME tab.
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Click Import.
Step 5: Import S/MIME Certificate
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Upload your S/MIME certificate file (.pfx or .p12).
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Enter the certificate password when prompted.
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Click OK to complete the import.
Step 6: Set Certificate as Default
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Select the newly imported certificate from the list.
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Click Set as Default from the top menu.
✅ Your S/MIME certificate is now installed and configured.
Verifying S/MIME Functionality
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Compose a new email and confirm the Sign options are available
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Send a test email to another S/MIME-enabled recipient
Troubleshooting
Issue: Certificate not listed after import
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Ensure the certificate password is correct
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Confirm the certificate format is supported (.pfx or .p12)
Issue: Recipient cannot decrypt messages
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Ensure the recipient’s public certificate is available
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Confirm both parties are using compatible S/MIME settings
Issue: Sign/Encrypt options are disabled
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Verify that the certificate is set as Default
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Check that Mail Encryption is enabled
Additional Notes
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S/MIME encryption requires certificate exchange between sender and recipient
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