Sitecore EXM customization - solution for https://sitecore.stackexchange.com/questions/17739/how-to-use-html-markup-in-customtokens-of-automated-messages

CustomFillEmail.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using Sitecore.Diagnostics;
using Sitecore.EDS.Core.Dispatch;
using Sitecore.EmailCampaign.Cm.Pipelines.SendEmail;
using Sitecore.EmailCampaign.Model.Message;
using Sitecore.EmailCampaign.Model.Web.Settings;
using Sitecore.ExM.Framework.Diagnostics;
using Sitecore.Modules.EmailCampaign;
using Sitecore.Modules.EmailCampaign.Core;
using Sitecore.Modules.EmailCampaign.Core.Crypto;
using Sitecore.Modules.EmailCampaign.Core.Personalization;
using Sitecore.Modules.EmailCampaign.Messages;
using System.Web;

namespace Sitecore.Exm.Custom.Pipelines
{
    public class CustomFillEmail
    {
        private readonly IStringCipher _cipher;
        private readonly ILogger _logger;
        private readonly PipelineHelper _pipelineHelper;
        private readonly EcmSettings _settings;
        
        public CustomFillEmail(IStringCipher cipher, ILogger logger, PipelineHelper pipelineHelper, EcmSettings settings)
        {
            _cipher = cipher;
            _logger = logger;
            _pipelineHelper = pipelineHelper;
            _settings = settings;
        }
        
        public void Process(SendMessageArgs args)
        {
            if (args.EcmMessage == null)
                return;
            this.GetMailFiller(args)?.FillEmail();
        }

        private MailFiller GetMailFiller(SendMessageArgs args)
        {
            HtmlMailBase ecmMessage1 = args.EcmMessage as HtmlMailBase;
            if (ecmMessage1 != null)
                // here we return custom mail filler
                return (MailFiller) new CustomHtmlMailFiller(ecmMessage1, args, _logger, _cipher, _pipelineHelper, _settings);
            MailMessageItem ecmMessage2 = args.EcmMessage as MailMessageItem;
            if (ecmMessage2 != null)
                return new MailMessageFiller(ecmMessage2, args, _cipher, _pipelineHelper, _settings);
            return null;
        }
    }

    public class CustomHtmlMailFiller : HtmlMailFiller
    {
        private readonly HtmlMailBase _htmlMailBase;

        public CustomHtmlMailFiller(HtmlMailBase message, SendMessageArgs args, ILogger logger, IStringCipher cipher, PipelineHelper pipelineHelper, EcmSettings settings) :
            base(message, args, logger, cipher, pipelineHelper, settings)
        {
            _htmlMailBase = message;
        }

        protected override void FillBody()
        {
            var utcNowStart = DateTime.UtcNow;
            var body = _htmlMailBase.Body;
            
            Email.ContentType = MessageContentType.Html;
            // decode all encoded content, very unsafe
            Email.HtmlBody = HttpUtility.Decode(_htmlMailBase.ReplaceTokens(body));
            Util.TraceTimeDiff("SetEmailHtmlBody(ReplaceTokens(Body))", utcNowStart);
            var utcNowEnd = DateTime.UtcNow;
            Email.PlainTextBody = _htmlMailBase.ReplaceTokens(_htmlMailBase.AlternateText);
            Util.TraceTimeDiff("SetEmailAltBody(ReplaceTokens(AlternateText))", utcNowEnd);
        }
    }

Patch.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
    <sitecore>
        <pipelines>
            <SendEmail>
                <processor type="Sitecore.EmailCampaign.Cm.Pipelines.SendEmail.FillEmail, Sitecore.EmailCampaign.Cm">
                    <patch:attribute name="type">
                        Sitecore.Exm.Custom.Pipelines.CustomFillEmail, Sitecore.Exm.Custom
                    </patch:attribute>
                </processor>
            </SendEmail>
        </pipelines>
    </sitecore>
</configuration>