Problems & more on Spanish PHP mail list

Hi. I just wanted to let you know that we are having problems in the Spanish mailing list. We are having a lot of [ERR] messages every time we send an email to the list, and somebody another email from cheap tickets saying ----Thank you for contacting Cheap Tickets. We have received your inquiry and are currently reviewing your request. Due to the popularity of our travel discounts, you may experience a delay.... So please is there any way to moderate this list or fix those problems? Thanks in advance.. Edwin. <? PHP ?>