Add watermark to emails in c# | https://docs.conholdate.com/net/add-watermarks-to-emails/

EmailAddWatermarkToAllAttachments.cs

TextWatermark watermark = new TextWatermark("Test watermark", new Font("Arial", 19));
EmailLoadOptions loadOptions = new EmailLoadOptions();
// Constants.InMessageMsg is an absolute or relative path to your document. Ex: @"C:\Docs\message.msg"
using (Watermarker watermarker = new Watermarker(Constants.InMessageMsg, loadOptions))
{
    EmailContent content = watermarker.GetContent<EmailContent>();
    foreach (EmailAttachment attachment in content.Attachments)
    {
        // Check if the attached file is supported by GroupDocs.Watermark
        IDocumentInfo info = attachment.GetDocumentInfo();
        if (info.FileType != FileType.Unknown && !info.IsEncrypted)
        {
            // Load the attached document
            using (Watermarker attachedWatermarker = attachment.CreateWatermarker())
            {
                // Add wateramrk
                attachedWatermarker.Add(watermark);

                // Save changes in the attached file
                attachedWatermarker.Save();
            }
        }
    }

    // Save changes
    watermarker.Save(Constants.OutMessageMsg);
}

Read the complete article: https://docs.conholdate.com/net/add-watermarks-to-emails/