I am not sure what Fastmail is so I will just assume that it is like Yahoo, Gmail, and such. As I said in my previous post, you are not going to be able to use the System.Web.Mail namespace for this project, if simply relays a mail message through a Mail server for you. It has no concept of mailboxes, aliases, etc.
The scope of this project is somewhat big since, i assume, you want to establish a service that allows people to come and set up email accounts and what not so you are going to have to seriously think about this and be prepared to devote many hours to the project!
The long and the short of it is, you are going to have to write code that sits between your web application and mail server that will be able to A)Send mail through the server from X users mail box B) Retrive a list of new mail messages and display those to the user C)Provide all other necessary functionality (changin password, creating mailing lists, etc.)
Again, I am assuming, you are going to get into some sockets programming with this but am not 100% sure as I have never done anything of this sort. hth.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^
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