If it does not work by assigning in html code, try to assign tooltip property in RowDataBound event in code behind file. Check the code below-
protected void Gridview1_RowDataBound(object sender, GridViewRowEventArgs e)
{
ImageButton lbtnSendMail = (ImageButton)e.Row.FindControl("lbtnSendMail");
lbtnSendMail.ToolTip = "Send Mail";
ImageButton ibtnDelete = (ImageButton)e.Row.FindControl("ibtnDelete");
ibtnDelete.ToolTip = "Delete";
}