Perhaps Your Account Does Not Exist on Mail Server?
If the mail server is remote to the computer you log into, it needs to have a way of knowing who you are. What I do is the lazy way and just set up a user account with the same name and password as the one I log into on my main Linux box. Then I set up qpopper POP3 server on the same system as Postfix is running on, and I can connect to it and get email with the same username/password as I log into my Linux workstation on.
There may be more elegant methods as using a centralized logon system such as NIS+ or LDAP, but I have had time to learn either of those. Anothing alternative would be to try and setting the desitation as usersname@workstation_ip_address ( e.g. email@example.com ) and see if that works for you.
That's all I have as far as ideas go.
Wait a sec, I haven't read your whole thread, but you are running a service to allow you to retrieve emails, right? Your options are using a file-based client to read email directly from the mail file ( difficult unless you are running the client on the same system as the Postfix mail server ), running a POP3 or IMAP server to 'serve' the email files to a POP3 or IMAP client, and that is all I can think of for ya. You cannot retrieve delivered emails from Postfix, all it does is send email out, and accept delivery of incoming emails from other SMTP servers.
I hope something in the email clarified things for you.