I have received a request from a current client for a quote on supplying his account with 4000 email addresses.

Let's leave in the middle for a second if I am going to give him what he wants. (spam complaints immediatly came to mind)

Is this dedicated server stuff or can it put put on the shared hosting server? Let's assume that at any one time 10% has their mailbox open and reads their mail every 10 minutes. That's 400 constant mail requests. What will this do to the server?

Current possibility of me accepting this clients new site: 1%

All your input is appreciated at this point. ;)

I would think a shared server with a low to medium load could handle it. I would also think twice, on if it's worth the trouble or not.

Are these going to be POP mail boxs or an IMAP mail boxs? If its IMAP.. it will use an awful lot more then POP. I would have them on a seperate server to be honest.

Craig

I suppose it all depends on what the client is willing to pay for the service? Most mail servers come with tools or third party add - ons that will allow you to keep the spammers at bay. You'll also have a TCO to protect you.
You should not require a big dedicated box to handle the load. The 2 most processor intensive things for the mail server will be processing incoming and out going messages (especially if you use an AV product), and A web based interface (Client) if you choose to use one. If you offer neither of these things, then the load will be minimal since the clients request for new messages generates little traffic. I do agree that the box should be dedicated though.
Be careful to limit mailbox sizes and attachment sizes, cause mail users will eat up bandwith and disk space very quickly if you don't keep them in check.
I am currently running I-mail with good success. If I can help you out, feel free to contact me off list.

HTH,