Looking For Recommended Email Marketing Service?

Hello,

Can anyone recommend a reliable service which offers auto-responders and allows importing of customers without a opt-in requirement?

I have a monthly newsletter subscription of ~21,000+ customers and am looking for another reliable email marketing service. I’ve been using InternetMailManager.com which charges ~$100/month, but lacks the updated features such as auto-responders and other services (although they don’t require opt-in for imported lists).

I’ve looked at GetResponse and AWeber which has a multitude of features, but require imported lists to opt-in (which would decrease the size of the mailing list and hassle current customers). All addresses are legitimate customers who have previously ordered from our website or subscribed using our website signup form.

Thanks for your help.