Try to rewrite it in this way and see what happens
for (int j = 0; j < ExcelColumnCount; j++)
{
string xmlTag = cleanedEmailTbl.Columns[j].ToString().Trim();
XML_TRANSACTIONDETAILS.AppendFormat("<{0}>{1}</{0}>", xmlTag, cleanedEmailTbl.Rows[i][j]);
}
Let us know.