GUI email client for WindowsLinux that uses local mailboxes

I'm looking for an email client that runs on Windows and Linux. I am looking for four things from this client:

1) It uses a GUI. 2) It reads it's mail from a local mailbox ie something in mbox, MH ... formats. I do not want any client which keeps it's own database. 3) I only want the GUI to do three things: 1) List the mail in a mailbox, 2) Let me read a mail message or delete a mail message, 3) Allow me to compose and send a email message ( via POP3 ) while saving it in a separate mailbox. 4) Manage different accounts. I want an account to have three parts, mail user name, mailbox location and format type ( if I'm using multiple formats which I haven't decided yet ), connection info for sending email for that account.

I want to be clear about this. I do not want profiles to be specified by what user is logged in. For example, I have a computer with two users Bob Smith and John Doe. I do not want the email to use one specific profile for Bob and another for John. Instead for example, Bob may have three different email accounts which are described by "BobSmith", "BobbySmith", and "RobertSmith". Each account would have a different mailbox. Same for John.

Thanks.