private void button1_Click(object sender, EventArgs e)
{
    string tinColumnId = panColumnIdTxtBox.Text.Trim('"');

    string emailColumnId = emailColumnIdTxtBox.Text.Trim('"');

    string excelFilePath = excelPathTxtBox.Text.Trim('"');

    string[] partAPathFiles = Directory.GetFiles(partAPathTxtBox.Text.Trim('"'));

    string[] partBPathFiles = Directory.GetFiles(partBPathTxtBox.Text.Trim('"'));

    if(partAPathFiles != null && partAPathFiles.Length == 0)
    {
        MessageBox.Show("No files found in Part A Path");
        return;
    }
    if(partBPathFiles != null && partBPathFiles.Length == 0)
    {
        MessageBox.Show("No files found in Part B Path");
        return;
    }
    if(!File.Exists(excelFilePath))
    {
        MessageBox.Show("Excel File not found");
        return;
    }

    using(ExcelEngine excelengine = new ExcelEngine())
    {
        IApplication application = excelengine.Excel;
        IWorkbook workbook = application.Workbooks.Open(excelFilePath);

        IWorksheet worksheet = workbook.Worksheets[0];

        IRange used = worksheet.UsedRange;

        int startRow = used.Row + 1;
        int endRow = used.LastRow;
        int statusColumn = used.LastColumn + 1;

        for(int row = startRow;row <= endRow;row++)
        {
            //Check the automation status of the tax payer report.
            if (worksheet.Range[row, statusColumn].Value == "Sent")
            {
                continue;
            }

            //Converting the Column Id to Column Index.
            IRange tinRange = worksheet[tinColumnId + row.ToString()];

            string tinID = worksheet.Range[row, tinRange.Column].Value;

            if(string.IsNullOrEmpty(tinID))
            {
                worksheet[row, statusColumn].Value = "TIN not found";
                continue;
            }

            IRange emailRange = worksheet[emailColumnId + row.ToString()];

            string emailId = worksheet.Range[row, emailRange.Column].Value;

            string partAPath = partAPathFiles.Where(x => x.Contains(tinID + "_")).FirstOrDefault();

            if (partAPath == null)
            {
                worksheet[row, statusColumn].Value = "Part A File not found";
                continue;
            }

            string partBPath = partBPathFiles.Where(x => x.Contains(tinID + "_")).FirstOrDefault();

            if (partBPath == null)
            {
                worksheet[row, statusColumn].Value = "Part B File not found";
                continue;
            }
            try
            {
                //Your mail ID.
                string from = "[email protected]";

                string subject = "Form 16 for the financial year 2022- 2023";

                string mailBody = "<p>Hi,</p>\r\n<p>Thank you for your kind cooperation</p>\r\n<p>We have attached the form 16 document pertaining to the financial year (2022-2023) tax deduction. Employees are requested to review and file their Income tax return before due date.</p>\r\n<p>The last date of filing of Income Tax return is July 31, 2023.</p>\r\n<p>For further assistance, please feel free to get back to us.</p>\r\n<p>Thanks and Regards,</p>\r\n<p>Accounts Team.</p>";

                SendEMail(from, emailId, subject, mailBody, partAPath, partBPath);

                worksheet[row, statusColumn].Value = "Sent";
            }
            catch(Exception ex){
                worksheet[row, statusColumn].Value = ex.ToString();
            }
        }

        Thread.Sleep(1000);

        // Saving the workbook containing successful and failed records.

        string filePath = Path.GetFileNameWithoutExtension(excelFilePath) + "_Updated_" + DateTime.Now.ToString().Replace(":", "-") + Path.GetExtension(excelFilePath); 

        workbook.SaveAs(filePath);

    }
    Close();

}

private static void SendEMail(string from, string recipients, string subject, string body, string attachmentPartA, string attachmentPartB)
{
    //Creates the email message.
    var emailMessage = new MailMessage(from, recipients);
    //Adds the subject for email.
    emailMessage.Subject = subject;
    //Sets the HTML string as email body.
    emailMessage.IsBodyHtml = true;
    emailMessage.Body = body;
    emailMessage.Attachments.Add(new Attachment(attachmentPartA));
    emailMessage.Attachments.Add(new Attachment(attachmentPartB));

    //Sends the email with prepared message.
    using (var client = new SmtpClient())
    {
        //Update your SMTP Server address here.
        client.Host = "outlook.office365.com";
        client.UseDefaultCredentials = false;
        //Update your email credentials here. Need to generate app password for your email ID.
        client.Credentials = new System.Net.NetworkCredential(from, "apppassword");
        client.Port = 587;
        client.EnableSsl = true;
        client.Send(emailMessage);                
    }
}