Set ReplyTo for newsletter issue

Use EmailSender.SendEmail global handler: You can hook into the low-level send process by registering a handler for EmailSender.SendEmail.Before in a global module.

using CMS;
using CMS.EmailEngine;
using CMS.Newsletters;

[assembly: RegisterModule(typeof(CustomEmailReplyToModule))]

public class CustomEmailReplyToModule : Module
{
    public CustomEmailReplyToModule() : base("CustomEmailReplyToModule") { }

    protected override void OnInit()
    {
        base.OnInit();

        // Hook BEFORE send
        EmailSender.SendEmail.Before += SendEmail_Before;
    }

    private void SendEmail_Before(object sender, EmailSenderEventArgs e)
    {
        var issueGuid = e.EmailMessage.EmailID; // Not directly the issue ID
        // The tricky part: identify if this is a newsletter email
        // and fetch the ReplyTo value from your custom field

        var issue = NewsletterIssueInfo.Provider.Get()
            .WhereEquals("IssueGUID", e.EmailMessage.EmailGuid)
            .FirstOrDefault();

        if (issue != null && !string.IsNullOrEmpty(issue.GetString("ReplyTo", "")))
        {
            e.EmailMessage.ReplyTo = issue.GetString("ReplyTo", "");
        }
    }
}

If you want to set the ReplyTo before messages get queued, hook into newsletter issue generation:

using CMS;
using CMS.Newsletters;

[assembly: RegisterModule(typeof(CustomNewsletterReplyToModule))]

public class CustomNewsletterReplyToModule : Module
{
    public CustomNewsletterReplyToModule() : base("CustomNewsletterReplyToModule") { }

    protected override void OnInit()
    {
        base.OnInit();

        NewsletterSender.SendIssue_Before += NewsletterSender_SendIssue_Before;
    }

    private void NewsletterSender_SendIssue_Before(object sender, NewsletterSenderEventArgs e)
    {
        var issue = e.Issue;
        if (issue != null && !string.IsNullOrEmpty(issue.GetString("ReplyTo", "")))
        {
            // This won't set Reply-To directly here,
            // but you can store it in e.Issue or a custom property
            // to later pick up in EmailSender.SendEmail.Before
        }
    }
}

Dear Vasu,

thank you for your quick response! I was trying to implement custom modules earlier, however, I cannot reproduce the way you suggested. EmailSender.SendEmail is a method, not an event that I can use for registering my own handler. I was also trying to use Newsletter specific events like NewsletterEvents.GenerateQueueItems.Before but such an event does not inlcude a way how to modify the generated emails. So, even though I understand the suggested approach and willing to use, I still can't find the correct event to register. Am I missing something?

The EmailSender class I have available in Kentico code (decompiled) is the following:

using CMS.Base;
using CMS.DataEngine;
using CMS.MacroEngine;

namespace CMS.EmailEngine
{
    public static class EmailSender
    {
        public static void SendEmail(EmailMessage message);

        public static void SendEmail(string siteName, EmailMessage message, string templateName, MacroResolver resolver, bool sendImmediately);

        public static void SendEmail(string siteName, EmailMessage message, bool sendImmediately = false);

        public static void SendEmailWithTemplateText(string siteName, EmailMessage message, EmailTemplateInfo template, MacroResolver resolver, bool sendImmediately);

        public static void SendMassEmails(EmailMessage message, string userIds, string roleIds, string groupIds, int siteId, bool sendToEveryone = false);

        public static void SendTestEmail(string siteName, EmailMessage message, SMTPServerInfo smtpServer);
    }
}

Try with this EmailEvents.Send.Before

Kentico provides EmailEvents which wrap the send pipeline. This is the lowest-level, correct way to intercept outgoing messages:

using CMS;
using CMS.EmailEngine;

[assembly: RegisterModule(typeof(CustomEmailReplyToModule))]

public class CustomEmailReplyToModule : Module
{
    public CustomEmailReplyToModule() : base("CustomEmailReplyToModule") { }

    protected override void OnInit()
    {
        base.OnInit();

        // hook into email events
        EmailEvents.Send.Before += Send_Before;
    }

    private void Send_Before(object sender, EmailEventArgs e)
    {
        var email = e.Email;

        // You can now inspect and modify the outgoing message
        if (email.EmailFormat == EmailFormatEnum.Html && string.IsNullOrEmpty(email.ReplyTo))
        {
            // Example: add Reply-To dynamically
            email.ReplyTo = "replyto@yourdomain.com";
        }
    }
}

Dear Vasu,

I am almost worried to write another bad news regarding your advice. There is no EmailEvents in our Kentico. I have prepared an overview of all the events we cas access (based on reflection and naming convention), it can be accessed on our website: https://www.nadacepartnerstvi.cz/specialni/avaliable-events. Bold is the class containing event members, italic are the fields and the field type is in brackets. From my point of view, I can't find anything that would allow me to access the Send event.

Sory for repeating question, but I still can't find a proper way to handle what we need.

The NewsletterEvents has the GenerateQueueItems event in which you should be able to get the generated emails and adjust them. If not, if you are sending the isues through email queue, I would try using the EmailInfo object events - then you would need to edit each email, so you need to be careful about the performance.