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.
-----BEGIN PRIVATE KEY-----
-----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.
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.