Check for TLS Support in Postfix
By running this command you can verify that TLS is supported by your version of Postfix.  Each of these parameters should exist.

# postconf -d | grep tls
lmtp_enforce_tls = no
lmtp_sasl_tls_security_options = $lmtp_sasl_security_options
lmtp_sasl_tls_verified_security_options = $lmtp_sasl_tls_security_options
lmtp_starttls_timeout = 300s
lmtp_tls_CAfile =
lmtp_tls_CApath =
lmtp_tls_cert_file =
lmtp_tls_dcert_file =
lmtp_tls_dkey_file = $lmtp_tls_dcert_file
lmtp_tls_enforce_peername = yes
lmtp_tls_exclude_ciphers =
lmtp_tls_key_file = $lmtp_tls_cert_file
lmtp_tls_loglevel = 0
lmtp_tls_mandatory_ciphers = medium
lmtp_tls_mandatory_exclude_ciphers =
lmtp_tls_mandatory_protocols = SSLv3, TLSv1
lmtp_tls_note_starttls_offer = no
lmtp_tls_per_site =
lmtp_tls_policy_maps =
lmtp_tls_scert_verifydepth = 5
lmtp_tls_secure_cert_match = nexthop
lmtp_tls_security_level =
lmtp_tls_session_cache_database =
lmtp_tls_session_cache_timeout = 3600s
lmtp_tls_verify_cert_match = hostname
lmtp_use_tls = no
milter_helo_macros = {tls_version} {cipher} {cipher_bits} {cert_subject} {cert_issuer}
smtp_enforce_tls = no
smtp_sasl_tls_security_options = $smtp_sasl_security_options
smtp_sasl_tls_verified_security_options = $smtp_sasl_tls_security_options
smtp_starttls_timeout = 300s
smtp_tls_CAfile =
smtp_tls_CApath =
smtp_tls_cert_file =
smtp_tls_dcert_file =
smtp_tls_dkey_file = $smtp_tls_dcert_file
smtp_tls_enforce_peername = yes
smtp_tls_exclude_ciphers =
smtp_tls_key_file = $smtp_tls_cert_file
smtp_tls_loglevel = 0
smtp_tls_mandatory_ciphers = medium
smtp_tls_mandatory_exclude_ciphers =
smtp_tls_mandatory_protocols = SSLv3, TLSv1
smtp_tls_note_starttls_offer = no
smtp_tls_per_site =
smtp_tls_policy_maps =
smtp_tls_scert_verifydepth = 5
smtp_tls_secure_cert_match = nexthop, dot-nexthop
smtp_tls_security_level =
smtp_tls_session_cache_database =
smtp_tls_session_cache_timeout = 3600s
smtp_tls_verify_cert_match = hostname
smtp_use_tls = no
smtpd_client_new_tls_session_rate_limit = 0
smtpd_enforce_tls = no
smtpd_sasl_tls_security_options = $smtpd_sasl_security_options
smtpd_starttls_timeout = 300s
smtpd_tls_CAfile =
smtpd_tls_CApath =
smtpd_tls_always_issue_session_ids = yes
smtpd_tls_ask_ccert = no
smtpd_tls_auth_only = no
smtpd_tls_ccert_verifydepth = 5
smtpd_tls_cert_file =
smtpd_tls_dcert_file =
smtpd_tls_dh1024_param_file =
smtpd_tls_dh512_param_file =
smtpd_tls_dkey_file = $smtpd_tls_dcert_file
smtpd_tls_exclude_ciphers =
smtpd_tls_key_file = $smtpd_tls_cert_file
smtpd_tls_loglevel = 0
smtpd_tls_mandatory_ciphers = medium
smtpd_tls_mandatory_exclude_ciphers =
smtpd_tls_mandatory_protocols = SSLv3, TLSv1
smtpd_tls_received_header = no
smtpd_tls_req_ccert = no
smtpd_tls_security_level =
smtpd_tls_session_cache_database =
smtpd_tls_session_cache_timeout = 3600s
smtpd_tls_wrappermode = no
smtpd_use_tls = no
tls_daemon_random_bytes = 32
tls_export_cipherlist = ALL:+RC4:@STRENGTH
tls_high_cipherlist = !EXPORT:!LOW:!MEDIUM:ALL:+RC4:@STRENGTH
tls_low_cipherlist = !EXPORT:ALL:+RC4:@STRENGTH
tls_medium_cipherlist = !EXPORT:!LOW:ALL:+RC4:@STRENGTH
tls_null_cipherlist = !aNULL:eNULL+kRSA
tls_random_bytes = 32
tls_random_exchange_name = ${config_directory}/prng_exch
tls_random_prng_update_period = 3600s
tls_random_reseed_period = 3600s
tls_random_source = dev:/dev/urandom

Once you have verified this information edit the main.cf file and add these lines:
smtpd_tls_cert_file=/etc/pki/dovecot/certs/dovecot.pem
smtpd_tls_key_file=/etc/pki/dovecot/private/dovecot.pem
smtpd_use_tls=yes

Note that the keys have been developed with the default location for Dovecot and the name  of dovecot.pem.  The important point is that the location be exactly the same for Postfix and Dovecot.  That is all you need to do for Postfix.  Reload Postfix.

By using the netstat command you will be able to verify the listening ports that dovecot is using.