Postfix SMTP Auth using Unix passwd

this seems to be a simple, stupid question, but I Googled at lot, and did not find anything. :-/

I have a Postfix SMTP and a Courier POP3. My Courier is using the native Linux user accounts (user/pass).

What I now want is to configure Postfix to use these accounts for authenticated, relayed SMTP mail as well, so my users can login to both POP3 and SMTP using the same credentials. I find a lot about SASL and a separate passwords file, LDAP, integrated MySQL solutions etc. etc., but that all too complex for my case.

I just want both Courier and Postfix to use the Linux user accounts to log in.

How do I do that?

This is my current Postfix main.cf:

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
home_mailbox = Maildir/
inet_interfaces = all
mailbox_size_limit = 1000000000
message_size_limit = 10000000
mydestination = mail.thomasjacob.de, localhost
myhostname = mail.thomasjacob.de
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 78.47.240.12
myorigin = /etc/mailname
readme_directory = no
recipient_delimiter = +
relayhost =
smtp_sasl_security_options = noanonymous
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
smtpd_recipient_restrictions = permit_tls_clientcerts permit_sasl_authenticated permit_mynetworks reject_non_fqdn_hostname reject_invalid_hostname reject_unauth_destination check_policy_service unix:private/tumgreyspf
smtpd_relay_restrictions = permit_tls_clientcerts permit_sasl_authenticated permit_mynetworks reject_non_fqdn_hostname reject_invalid_hostname reject_unauth_destination check_policy_service unix:private/tumgreyspf
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls=yes
tumgreyspf_time_limit = 3600
virtual_alias_domains = thomasjacob.de
virtual_alias_maps = hash:/etc/postfix/virtual