For your newsletter-mailings, I mean. Please recommend one. I'll tell you who NOT to use : Yahoo's Got Campaign. Lousy service. Pitiful customer service. Stay away or throw away your money. ----end of furious ramble-----
I don't use one but I am subscribed to a bunch of newsletters, about three or four of them use aweber: http://www.aweber.com/
I use 1-2-All for the management platform and host it on a dedicated server which has been optimised for the rapid sending of emails. It punches about 20,000 mails an hour now.