How to remove all the null elements inside a generic list in one go?

I do not know of any in-built method, but you could just use linq:

parameterList = parameterList.Where(x => x != null).ToList();

You'll probably want the following.

List<EmailParameterClass> parameterList = new List<EmailParameterClass>{param1, param2, param3...};
parameterList.RemoveAll(item => item == null);