LDAP+nss_ldap+squid
 

The goal is to create a single authentication system for smtp, imap, proxy, vpn, local login, pki. At the moment I stopped at openldap, although the option with the 389-ds is also considered. Is it possible to create user user1, who would be a maildir, separate from home at mail server? Perhaps for security purposes you must create two: one for mail, another for the rest. But is it possible to provide the same password will be created if the two separate users?

Virtual users
 

Since most people use IMAP or webmail these days to access their mailbox, I recommend using virtual users. This gives you more flexibility and you won't have headaches because of permission problems on maildirs. If you want to use the same passwords, you just need to copy the userPassword field to the mail entry.