SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-query-message-templates

QueryMessageBody.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Telerik.Sitefinity.Modules.Newsletters;
using Telerik.Sitefinity.Newsletters.Model;

namespace Telerik.Sitefinity.Documentation.CodeSnippets.DevGuide.SitefinityEssentials.Modules.EmailCampaigns.MessageBodies
{
    public partial class EmailCampaignsSnippets
    {
        public MessageBody QueryMessageBody(Guid id)
        {
            NewslettersManager manager = NewslettersManager.GetManager();
            MessageBody messageBody = manager.GetMessageBodies().Where(b => b.Id == id).SingleOrDefault();

            return messageBody;
        }
    }
}