I was previously with them and horrible service. The service was unreliable at best and when there was a problem, the foriegn support staff would gladly beat around the bush and offer to shuffle me from server to server to server causing much downtime and very unreliable service. Of course this is all when they were oblivious to the fact of a problem existing all together.
I must say, I switched was ready to switch to RSNET, who's services were offline, and had been offline for a day or 2 already. Several Google searches and I found this wasn't the first time for them. I then decided to switch to MDD and haven't looked back since the year and a half I've been with them. Just this morning I commited to another 2 years.
They offer very professional services and a more than reasonable price. I've wanted sometime to switch to their semi-dedicated services. He's told me more than once that I wasn't at the level of needing to and it wasn't time yet, but they had a special which I couldn't turn down running so I did it anyways.
If you're looking for mail servers, be sure to check their server ip against spam blacklists. with some email providers, if too many people mark your mail as spam, nobody on the service might get your mail.
If the provider provide 30-day money back guarantee, try using an account with them to see if you get it.
Contact the web host before doing any mass mailing, I know a lot of web host are very strick when it comes down to mass mailing.
Also you might want to invest in a dedicated server / VPS.
Personally I would find it hard to recommend a dedicated server or VPS especially when the OP hasn't posted what exactly they plan to do with the "mail server". If it's for mass mailing then a VPS is probably a good idea but that's not always what a mail server is used for... Sometimes it's simply used to handle the mail for an individual or a business without needing much of a "mass mailing" capability.
I think you will find numerous helpful reviews here for both companies. Also make sure to Google each as well. Perhaps search terms such as "...hostname sucks" etc. That term will often produce some interesting results.