===Background=
DNN introduced the messaging and notification centre in
6.2.0 , where users can send and receive messages and notifications. Whilst this is similar in concept to email, it his it's own API to support sending to lists of users or/and roles (including social groups).
To send a simple messages to the current user you can use code such as :
DotNetNuke.Services.Social.Messaging.Message message = new Message();
List<UserInfo> users = new List<DotNetNuke.Entities.Users.UserInfo>();
users.Add(UserController.GetUserById(INTEGER, INTEGER));
message.Body = STRING;
message.Subject = STRING;
MessagingController.Instance.SendMessage(message, null, users, null, this.UserInfo);