There's nothing documented, but here's how you do it:

  // fetch the email config:
  Sage.MMSAdmin.DBSchema.Messaging.EmailServerConfiguration config = 
        Sage.MMSAdmin.DBLayer.ServerConfigurations.GetEmailServerConfiguration();

  //determine the encryption key:
  SecureString encryptionKey = Sage.MMSAdmin.Util.SecureStringHelper.StringToSecureString(
        config._EmailServerConfiguration[0].UserLogin + config._EmailServerConfiguration[0].ServerName);

  //decrypt the pwd
  string plainTextPwd = 
        Sage.MMSAdmin.Cryptography.SymmetricEncryption.DecryptText(
                    encryptionKey, config._EmailServerConfiguration[0].UserPassword);

Hi Chris, which dll file that I need to import as reference?

You'll need:

Sage.MMSAdmin.DbLayer.dll

Sage.MMSAdmin.DBSchema.dll

Sage.MMSAdmin.Util.dll

..and there might be an indirect dependency on Sage.Web.Services.Framework.dll, I can't remember for sure.

You won't find all of those in the loose assmblies that you get as part of the SDK.  You'll have to source some of them from the \bin folders of the Sage 200 Services that you'll find under inetpub.

ok noted, I will try it out. Thank you