Watcher : Email Action : How to use attributes with special characters in the name

[Lalit_Negi]

I have a query result where the source has some attributes with speical chars in name

ctx.payload.hits.hits[0]._source['@msg']

and I want to send these attributes in a watcher email action.

however these kind of attributes never work under the transform => script => source section
I tried many things like replacing the attribute with _ etc , using a varible like
def attr = '@msg'
but nothing works

below is a sample example (formatted the source string for easy understanding

    "transform": {
        "script": {
          "source": "ctx.payload.transform = [];
                    def document = [];     
                    for (int j = 0; j < ctx.payload.hits.hits.length; j++) {
                      def src = ctx.payload.hits.hits[j]._source;    
                      document = ['details':  src['@mt'] ];  
                      ctx.payload.transform.add(document); 
                    }     
                    return ctx.payload.transform;",
          "lang": "painless"
        }
      }

Kindly suggest any way to access these attribute in the watchers email painless script please.

Thanks in advance