Mailman, postfix troubles

I am trying to think of a way to describe this clearly. I'm sure you will all
let me know if I have muddied things. Here we go.

I am added Mailman to my email server. I can bring up the admin interface and
have supposedly created one of the email lists. However, no mail comes out.
I think I know why I just don't know how to solve it.

Before Mailman: Incoming

ISPMailBox--> Fetchmail--> localhost:25--> spamassasin--> postfix-->
UserMailBox--> POP--> UserClient

Before Mailman: Outgoing

UserClient--> ISPSMTPServer

The outgoing is possible because all the client email accounts also exist at
the ISP. Each client logs directly to the ISP SMTP server to send mail out.
This is good because the ISP will not allow my mail server to use port 25 out
of their system. You know, the spam thing.

After Mailman: Incoming

ISPMailBox--> Fetchmail--> localhost:25--> spamassasin--> postfix-->
UserMailBox--> Mailman

I don't know if this is working. I don't know Mailman well enough yet to
determine if it picks up the email in the list's user mail box. I assume
that it is.

After Mailman: Outgoing

Mailman--> postfix--> nowhere

The mail is not getting out. I don't know how to make postfix log into my
ISP's SMTP sever to send anything out.

Again, the mail list user has an email account at the ISP. Also, the final
solution needs to allow the non-list email accounts to still send and recive
mail.

I am reading online info in ernest. I am a newbie email admin with just
enough knowledge to be dangerous. As I read maybe one of you can point out a
better way.

Alan