code: public Configuration addDefaultParameter(String name, String value) {
          defaultParameters.add(name, value);
          return this;
      }
docs: Adds a new value to the specified default parameter.
  <p>
  This is only used if the parameter is not specified when building
  the specific mail.
  <p>
  Please note that parameters are multivalued. This method adds a new
  value. To set a new value you need to clear the default parameter first.
  
  @param name the name of the parameter
  @param value the new value to add to the parameter
  @return this configuration
  @see #clearDefaultParameter(String)