spam proof bulk mail

We are a company who require an expert in the field of email marketing. We need have a client who had generated signups to through their website and all the lists are legitimate. We need to send out about 50,000 emails at a time and want to ensure that the emails do not get put in the receivers SPAM box. Our current server does not support large email sendouts and sending the mail locally causes the mail to be SPAMMED, we need advice from experts in the field. We need an expert in the field. We also wish to hear from you regarding ways of adding to the sendout list through legitiamte routes. We would like to hear of your track record in the field and know what you can offer.