SSL services are more popular for modern network usage. Secure your data from A to B is very important issue you couldn't ignore.

To replace default certifcate and private key of RaidenMAILD, you need apply a new certificate first, no matter it is free or paid.

FREE SSL CERTIFICATE: You could try ZEROSSL

FREE SSL CERTIFICATE: You could try CertBot

PAID SSL CERTIFICATE: You could try COMODO SSL , if you need to generate CSR, try CSRGENERATOR

PS: Make sure COPY PRIVATE KEY CONTENT to TEXT FILE, and name it to privkey.pem, we need this file later.

Example:
-----BEGIN PRIVATE KEY-----
MIIEvgIBADANBgkqhkiG9w0BAQEFAASCBKgwggSkAgEAAoIBAQDUF4uaR0vPRQ7D
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rct88LuVNg+E7SK4gz51mAbuCDu08QWc6H4cy8x48wr+n4HTLbfK/qMQrVO1ZsZU
C7tte0LvlYjEfko+8zwilDN1wTjWgPszY5wQa4CrpQgkjpOBiZj36tdWn8O+uIO1
67XYtosxrZdDUMsApQyfPtRx
-----END PRIVATE KEY-----

After you got certificate files, it should be something like these.

For this example, you need to change these filenames for RaidenMAILD usage.
1) Change www_raidenmaild_com.crt to cert.pem.
2) Change My_CA_Bundle.ca-bundle to cacert.pem

After the steps above are done, you already got 3 files ready to use.
They are cert.pem, cacert.pem and privkey.pem.
Just copy them to <RaidenMAILD>\SSL folder to overwrite old files and restart your service to take effect.