sendmail for Solaris 2.x

1. problem in unstanding sendmail/cf SMTP auth (Solaris 8 Sendmail 8.10) Please Explain

I want to set up a mail server like a provider wants to have.
The machine is an INTEL SOLARIS 8, sendmail is 8.10 like on the
companion CD.
It shall accept mails for local domains but "external" users who wants
to SMTP shall be allowed to do this if they are "customers" which they
can proof with user password.
I have problems with the following part of the documentation.

sendmail/cf says:
Requiring SMTP AUTH for all mails is in general a bad idea, because
then you cannot receive mails from other users (since the cannot
authenticate). So you must do this only on a server that is solely
intended for your own users to send mail, not for a publically
advertised (via MX records) server.

How to do this?
My mailserver is public via MX-records. Outgoing mails (mails to all
domains different from the local ones) shall require SMTP-auth,
mails to the local domains must be received.
Mails from outside must come in without SMTP-auth, because other
mailservers can't authorize themselves which should be clear- so the
local domains are allowed to relay to. So far i understood all this i
CANNOT remove them from the list of local domains (class R).
Or is there another possible solution?

Summary to clarify the problem:
The collected facts and functionallity is:

1.)Domain "mydomain1" and "mydomain2" shall receive mail from all the
world, (works now)
2.)customers shall be able to receive it with POP3 (Qpopper is
running) ,(works now)
3.)all the world shall be able to send mails to this server for the
"mydomains" (works now)
4.)local users shall be able to send mail to all the world (works now
with ip-restriction)(optional with no auth ..)

new item:
5.)(Our) users being outside the local network shall be able to send
mail to all the world after authorisation (does not work until now)

For "3.)" to work, as i understood, relaying to local domains must be
allowed absolutely unrestricted, because other mailserver log in on
port 25 and SMTP to the local domain.

Has someone an idea how to configure that all together?

Is it possible what i want to configure or do i have to decide between
auth7 and access-file?

Is the standard-Sendmail from the companion CD ready for SMTP auth or
do i have to rebuild????

Thank you , desperately waiting for hints

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