var sendingManagerFactory =
ServiceLocator.ServiceProvider.GetService<ISendingManagerFactory>();
var campaignService = ServiceLocator.ServiceProvider.GetService<IExmCampaignService>();

MessageItem emailCampaign = campaignService.GetMessageItem(emailCampaignId);

ISendingManager sendingManager = sendingManagerFactory.GetAsyncSendingManager(emailCampaign);

sendingManager.SendMessage();