.NET AddIn Development

I am writing a program that allows developers to write AddIn for it, and I would like the developers to be able to hook up to events that occur in the program.

My code won't compile because I can't declare a delegate in the IMyProgram interface.

So, I suppose it's more of a design issue. How could you get the interface passed to AddIn so that AddIn can connect to program events?

[AddInContract]
    public interface IMyProgramAddInContract : IContract {

        /// <summary>
        /// Initializes AddIn
        /// </summary>
        void Init(IMyProgram instance);

        System.Drawing.Image AddInIcon { get; }

        String DisplayName { get; }

        String Description { get; }
    }

    [AddInContract]
    public interface IMyProgram : IContract {

        public delegate EventHandler EmailEventHandler(object sender, EmailEventArgs args);

        public event EmailEventHandler BeforeCheck;
        public event EmailEventHandler AfterCheck;
        public event EmailEventHandler EmailDownloaded;
        public event EmailEventHandler OnProcessMessage;

    }

    [AddInBase]
    public class EmailEventArgs : EventArgs {

        public override string ToString() {
            return "todo";
        }
    }