Install packages using FreeBSD port.

1. Install postfix

# Options for postfix-2.11.0,1
_OPTIONS_READ=postfix-2.11.0,1
_FILE_COMPLETE_OPTIONS_LIST=BDB CDB INST_BASE LDAP_SASL LMDB MYSQL NIS OPENLDAP PCRE PGSQL SASL2 SPF SQLITE TEST TLS VDA DOVECOT DOVECOT2 SASLKRB5 SASLKMIT
OPTIONS_FILE_SET+=BDB
OPTIONS_FILE_SET+=CDB
OPTIONS_FILE_UNSET+=INST_BASE
OPTIONS_FILE_SET+=LDAP_SASL
OPTIONS_FILE_UNSET+=LMDB
OPTIONS_FILE_UNSET+=MYSQL
OPTIONS_FILE_UNSET+=NIS
OPTIONS_FILE_SET+=OPENLDAP
OPTIONS_FILE_SET+=PCRE
OPTIONS_FILE_UNSET+=PGSQL
OPTIONS_FILE_SET+=SASL2
OPTIONS_FILE_UNSET+=SPF
OPTIONS_FILE_UNSET+=SQLITE
OPTIONS_FILE_SET+=TEST
OPTIONS_FILE_SET+=TLS
OPTIONS_FILE_UNSET+=VDA
OPTIONS_FILE_UNSET+=DOVECOT
OPTIONS_FILE_UNSET+=DOVECOT2
OPTIONS_FILE_UNSET+=SASLKRB5
OPTIONS_FILE_UNSET+=SASLKMIT

2. install openldap

# Options for openldap-client-2.4.38
_OPTIONS_READ=openldap-client-2.4.38
_FILE_COMPLETE_OPTIONS_LIST=FETCH
OPTIONS_FILE_UNSET+=FETCH

3. install cyrus-sasl2

# Options for cyrus-sasl-2.1.26_4
_OPTIONS_READ=cyrus-sasl-2.1.26_4
_FILE_COMPLETE_OPTIONS_LIST=ALWAYSTRUE AUTHDAEMOND KEEP_DB_OPEN  OBSOLETE_CRAM_ATTR BDB MYSQL PGSQL SQLITE2 SQLITE3 CRAM DIGEST LOGIN NTLM OTP PLAIN SCRAM
OPTIONS_FILE_UNSET+=ALWAYSTRUE
OPTIONS_FILE_SET+=AUTHDAEMOND
OPTIONS_FILE_UNSET+=KEEP_DB_OPEN
OPTIONS_FILE_SET+=OBSOLETE_CRAM_ATTR
OPTIONS_FILE_UNSET+=BDB
OPTIONS_FILE_UNSET+=MYSQL
OPTIONS_FILE_UNSET+=PGSQL
OPTIONS_FILE_UNSET+=SQLITE2
OPTIONS_FILE_UNSET+=SQLITE3
OPTIONS_FILE_SET+=CRAM
OPTIONS_FILE_SET+=DIGEST
OPTIONS_FILE_SET+=LOGIN
OPTIONS_FILE_SET+=NTLM
OPTIONS_FILE_SET+=OTP
OPTIONS_FILE_SET+=PLAIN
OPTIONS_FILE_SET+=SCRAM

4. install cyrus-sasl2-saslauthd

# Options for cyrus-sasl-saslauthd-2.1.26
_OPTIONS_READ=cyrus-sasl-saslauthd-2.1.26
_FILE_COMPLETE_OPTIONS_LIST=BDB HTTPFORM OPENLDAP
OPTIONS_FILE_SET+=BDB
OPTIONS_FILE_SET+=HTTPFORM
OPTIONS_FILE_SET+=OPENLDAP

5. install postfwd

# Options for postfix-postfwd-1.32_1
_OPTIONS_READ=postfix-postfwd-1.32_1
_FILE_COMPLETE_OPTIONS_LIST=DOCS EXAMPLES POSTFWD2
OPTIONS_FILE_SET+=DOCS
OPTIONS_FILE_SET+=EXAMPLES
OPTIONS_FILE_UNSET+=POSTFWD2

6. configure /usr/local/lib/sasl2/smtpd.conf

log_level: 3
pwcheck_method: saslauthd
mech_list: PLAIN LOGIN

7. configure /usr/local/etc/saslauthd.conf

ldap_servers:
ldap_bind_dn:
ldap_bind_pw:
ldap_search_base:
ldap_auth_method: ssha
ldap_time_limit: 4
ldap_filter:

8. configure /usr/local/etc/postfix/main.cf

mtpd_sasl_auth_enable = yes
smtpd_sasl_local_domain =
smtpd_sasl_authenticated_header = yes
broken_sasl_auth_clients = yes
smtpd_sasl_path = smtpd
smtp_sasl_type = cyrus
smtpd_sasl_security_options = noanonymous

smtpd_recipient_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_unauth_destination

9. configure /usr/local/etc/postfwd.conf

id=RULE001
 sasl_username=~/^(\S+)$/
 action=rcpt(sasl_username/200/3600/DEFER Too much emails for $$sasl_username)

10. configure /etc/rc.conf

postfix_enable="YES"
saslauthd_enable="YES"
saslauthd_flags="-a ldap"
postfwd_enable="YES"

Reference: http://ashterix.blogspot.com/2008/10/freebsd-postfix-sasl-openldap.html