How did you send the testing email?

Port 993 is pop3.
Please double check your Thunderbird setting, do you use port 587 with starttls to send email? Also, show me output of command "postconf -n", and the lines define "submission" transport in /etc/postfix/master.cf.

alias_database = hash:/etc/postfix/aliases
alias_maps = hash:/etc/postfix/aliases
allow_min_user = no
allow_percent_hack = no
biff = no
body_checks = pcre:/etc/postfix/body_checks.pcre
bounce_queue_lifetime = 4h
broken_sasl_auth_clients = yes
command_directory = /usr/sbin
config_directory = /etc/postfix
content_filter = smtp-amavis:[127.0.0.1]:10024
daemon_directory = /usr/libexec/postfix
data_directory = /var/lib/postfix
debug_peer_level = 2
debugger_command = PATH=/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin ddd $daemon_directory/$process_name $process_id & sleep 5
delay_warning_time = 0h
disable_vrfy_command = yes
dovecot_destination_recipient_limit = 1
enable_original_recipient = no
header_checks = pcre:/etc/postfix/header_checks
home_mailbox = Maildir/
html_directory = no
inet_interfaces = all
inet_protocols = ipv4
lmtp_tls_mandatory_protocols = !SSLv2 !SSLv3
lmtp_tls_protocols = !SSLv2 !SSLv3
mail_owner = postfix
mailbox_command = /usr/libexec/dovecot/deliver
mailbox_size_limit = 104857600
mailq_path = /usr/bin/mailq.postfix
manpage_directory = /usr/share/man
maximal_backoff_time = 4000s
maximal_queue_lifetime = 4h
message_size_limit = 104857600
minimal_backoff_time = 300s
mydestination = $myhostname, localhost, localhost.localdomain
mydomain = hostname.my_domain_name.tld
myhostname = hostname.my_domain_name.tld
mynetworks = 127.0.0.1, a_private_subnet/24
mynetworks_style = host
myorigin = hostname.my_domain_name.tld
newaliases_path = /usr/bin/newaliases.postfix
proxy_read_maps = $canonical_maps $lmtp_generic_maps $local_recipient_maps $mydestination $mynetworks $recipient_bcc_maps $recipient_canonical_maps $relay_domains $relay_recipient_maps $relocated_maps $sender_bcc_maps $sender_canonical_maps $smtp_generic_maps $smtpd_sender_login_maps $transport_maps $virtual_alias_domains $virtual_alias_maps $virtual_mailbox_domains $virtual_mailbox_maps $smtpd_sender_restrictions
queue_directory = /var/spool/postfix
queue_run_delay = 300s
readme_directory = /usr/share/doc/postfix-2.11.0/README_FILES
recipient_bcc_maps = proxy:mysql:/etc/postfix/mysql/recipient_bcc_maps_user.cf, proxy:mysql:/etc/postfix/mysql/recipient_bcc_maps_domain.cf
recipient_delimiter = +
relay_domains = $mydestination, proxy:mysql:/etc/postfix/mysql/relay_domains.cf
sample_directory = /usr/share/doc/postfix-2.11.0/samples
sender_bcc_maps = proxy:mysql:/etc/postfix/mysql/sender_bcc_maps_user.cf, proxy:mysql:/etc/postfix/mysql/sender_bcc_maps_domain.cf
sendmail_path = /usr/sbin/sendmail.postfix
setgid_group = postdrop
smtp-amavis_destination_recipient_limit = 1
smtp_data_init_timeout = 240s
smtp_data_xfer_timeout = 600s
smtp_tls_CAfile = $smtpd_tls_CAfile
smtp_tls_loglevel = 0
smtp_tls_mandatory_protocols = !SSLv2 !SSLv3
smtp_tls_note_starttls_offer = yes
smtp_tls_protocols = !SSLv2 !SSLv3
smtp_tls_security_level = may
smtpd_data_restrictions = reject_unauth_pipelining
smtpd_end_of_data_restrictions = check_policy_service inet:127.0.0.1:7777
smtpd_helo_required = yes
smtpd_helo_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_non_fqdn_helo_hostname, reject_invalid_helo_hostname, check_helo_access pcre:/etc/postfix/helo_access.pcre
smtpd_recipient_restrictions = reject_unknown_recipient_domain, reject_non_fqdn_recipient, reject_unlisted_recipient, check_policy_service inet:127.0.0.1:7777, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, reject_rbl_client b.barracudacentral.org, reject_rbl_client bl.spamcop.net, reject_rbl_client cbl.abuseat.org, reject_rbl_client zen.spamhaus.org
smtpd_reject_unlisted_recipient = yes
smtpd_reject_unlisted_sender = yes
smtpd_sasl_auth_enable = yes
smtpd_sasl_local_domain =
smtpd_sasl_path = private/dovecot-auth
smtpd_sasl_security_options = noanonymous
smtpd_sasl_type = dovecot
smtpd_sender_login_maps = proxy:mysql:/etc/postfix/mysql/sender_login_maps.cf
smtpd_sender_restrictions = reject_unknown_sender_domain reject_non_fqdn_sender reject_unlisted_sender permit_mynetworks permit_sasl_authenticated check_sender_access pcre:/etc/postfix/sender_access.pcre
smtpd_tls_CAfile = /etc/pki/tls/certs/my_domain_name_tld_2016_chained_bundle.pem
smtpd_tls_auth_only = yes
smtpd_tls_cert_file = /etc/pki/tls/certs/my_domain_name_tld_2016_chained_bundle.pem
smtpd_tls_dh1024_param_file = /etc/pki/tls/dhparams.pem
smtpd_tls_key_file = /etc/pki/tls/private/my_domain_name.tld.key
smtpd_tls_loglevel = 1
smtpd_tls_mandatory_exclude_ciphers = aNULL, eNULL, EXPORT, DES, RC4, MD5, PSK, aECDH, EDH-DSS-DES-CBC3-SHA, EDH-RSA-DES-CDC3-SHA, KRB5-DE5, CBC3-SHA
smtpd_tls_mandatory_protocols = !SSLv2 !SSLv3
smtpd_tls_protocols = !SSLv2 !SSLv3
smtpd_tls_security_level = may
swap_bangpath = no
tls_random_source = dev:/dev/urandom
transport_maps = proxy:mysql:/etc/postfix/mysql/transport_maps_user.cf, proxy:mysql:/etc/postfix/mysql/transport_maps_domain.cf
unknown_local_recipient_reject_code = 550
virtual_alias_domains =
virtual_alias_maps = proxy:mysql:/etc/postfix/mysql/virtual_alias_maps.cf, proxy:mysql:/etc/postfix/mysql/domain_alias_maps.cf, proxy:mysql:/etc/postfix/mysql/catchall_maps.cf, proxy:mysql:/etc/postfix/mysql/domain_alias_catchall_maps.cf
virtual_gid_maps = static:2000
virtual_mailbox_base = /var/vmail
virtual_mailbox_domains = proxy:mysql:/etc/postfix/mysql/virtual_mailbox_domains.cf
virtual_mailbox_maps = proxy:mysql:/etc/postfix/mysql/virtual_mailbox_maps.cf
virtual_minimum_uid = 2000
virtual_transport = dovecot
virtual_uid_maps = static:2000

Hi Zhang,

Isn't Port 993 for IMAP via SSL and Port 995 is Pop3 via SSL?

When I looked at Thunderbird and I was the server setting which is using the sever name smpt.domain_name.tld and port.  This is for the imap connections and not outgoing.  This was reinstall of the Thunderbird post OS install and not sure why the reference was set to smtp in their GUI setup but they're the same IP currently.  So did not detect this originally...

Outgoing is set to smtp.domain_name.tld and the port is 587 with StartTLS.

Still no DKIM key present in sent email.