Sounds like you used a wrong smtp server address in your web application, or port 587 is not reachable to your web application.

Can you connect to port 587 on your laptop?

And this is my postconf -f results

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
command_directory = /usr/sbin
compatibility_level = 2
content_filter = smtp-amavis:[127.0.0.1]:10024
daemon_directory = /usr/lib/postfix/sbin
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
disable_vrfy_command = yes
dovecot_destination_recipient_limit = 1
enable_original_recipient = no
header_checks = pcre:/etc/postfix/header_checks
inet_interfaces = all
inet_protocols = all
lmtp_tls_mandatory_protocols = !SSLv2 !SSLv3
lmtp_tls_protocols = !SSLv2 !SSLv3
mail_owner = postfix
mailq_path = /usr/bin/mailq
message_size_limit = 15728640
mlmmj_destination_recipient_limit = 1
mydestination = $myhostname, localhost, localhost.localdomain
mydomain = mail.mailfrom-domain.ml
myhostname = mail.mailfrom-domain.ml
mynetworks = 127.0.0.1 [::1]
myorigin = mail.mailfrom-domain.ml
newaliases_path = /usr/bin/newaliases
postscreen_access_list = permit_mynetworks cidr:/etc/postfix/postscreen_access.cidr
postscreen_blacklist_action = drop
postscreen_dnsbl_action = drop
postscreen_dnsbl_reply_map = texthash:/etc/postfix/postscreen_dnsbl_reply
postscreen_dnsbl_sites = zen.spamhaus.org=127.0.0.[2..11]*3 b.barracudacentral.org=127.0.0.2*2
postscreen_dnsbl_threshold = 2
postscreen_dnsbl_whitelist_threshold = -2
postscreen_greet_action = drop
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 $sender_dependent_relayhost_maps
queue_directory = /var/spool/postfix
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
sender_bcc_maps = proxy:mysql:/etc/postfix/mysql/sender_bcc_maps_user.cf proxy:mysql:/etc/postfix/mysql/sender_bcc_maps_domain.cf
sender_dependent_relayhost_maps = proxy:mysql:/etc/postfix/mysql/sender_dependent_relayhost_maps.cf
sendmail_path = /usr/sbin/sendmail
setgid_group = postdrop
smtp-amavis_destination_recipient_limit = 1
smtp_tls_CAfile = $smtpd_tls_CAfile
smtp_tls_CApath = /etc/ssl/certs
smtp_tls_loglevel = 1
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 check_helo_access pcre:/etc/postfix/helo_access.pcre reject_non_fqdn_helo_hostname reject_unknown_helo_hostname
smtpd_recipient_restrictions = reject_non_fqdn_recipient reject_unlisted_recipient check_policy_service inet:127.0.0.1:7777 permit_mynetworks permit_sasl_authenticated reject_unauth_destination
smtpd_reject_unlisted_recipient = yes
smtpd_reject_unlisted_sender = yes
smtpd_sasl_path = private/dovecot-auth
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/letsencrypt/live/mail.mailfrom-marketingleo.ml/chain.pem
smtpd_tls_CApath = /etc/ssl/certs
smtpd_tls_cert_file = /etc/letsencrypt/live/mail.mailfrom-marketingleo.ml/cert.pem
smtpd_tls_dh1024_param_file = /etc/ssl/dh2048_param.pem
smtpd_tls_dh512_param_file = /etc/ssl/dh512_param.pem
smtpd_tls_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_key_file = /etc/letsencrypt/live/mail.mailfrom-domain.ml/privkey.pem
smtpd_tls_loglevel = 1
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_maillist.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
ubuntu@mail:~/iRedMail-0.9.8$

Hi Zhang,

yes my application will support TLS connection (screenshots are below)

TLS: http://prntscr.com/kid0jf

TYPE: https://prnt.sc/kid0ys

And sorry Zhang, Now emails are going through 587 port. Because I had not opened 587 in AWS ec2 security group I am very sorry that was the mistake from my end. Now from my application emails are going fine through 587. And thank you very much for your support.

Now I have one small doubt, Think that I have many different domains entered in Iredmail panel if I want to get DKIM records for each of the domain, then how can I do that.  (I have got for main domain)