Mail server
Accounts can do mail if a home dir is created (Dovecot LDA requires this) and they are in the mailusers group (/etc/security/access.conf)
Local accounts should have uid <1000 and need to be specifically listed in the exim conf (special_user:) don't forget to update-exim4.conf and reload exim.
There's a crontabed script do sync filesystem ACLs with IMAP ACLs called migrate_acl.pl