Q:

Sending email inside a docker container to hosts smtp running postfix

On a server I run both docker images and a postfix smtp server. The smtp server is accesible via localhost and is not installed in any sort of container.
The problem is that I can send email via terminal using sendemail outside of any container but I cannot send email via terminal running sendemail inside of any container.
The postfix configuration is the following:
# See /usr/share/postfix/main.cf.dist for a commented, more complete version

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtpd_tls_cert_file=/etc/letsencrypt/live/ellak.org/fullchain.pem
smtpd_tls_key_file= /etc/letsencrypt/live/ellak.org/privkey.pem

smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = guest8.ellak.gr
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = mail.ellak.org, guest8.ellak.gr, localhost.ellak.gr, localhost

#Virtual alias domains
virtual_alias_domains = ellak.org

# RELAY Options
relayhost = [mail1.ellak.gr]:587
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

smtp_use_tls = yes
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
local_recipient_maps = proxy:unix:passwd.byname $alias_maps
smtp_tls_note_starttls_offer = yes

#Network Access Options

mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 172.17.0.0/16
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
relay_domains = *

Inside the container I run:

sendemail -s 172.17.0.1:25 -f no-reply@ellak.org -t 
  katanophoros@gmail.com -u Hello -m Hello

And outputs the following error:
Nov 27 19:51:14 7e56b4e22e09 sendemail[1988]: WARNING => The recipient <katanophoros@gmail.com> was rejected by the mail server, error follows:
Nov 27 19:51:14 7e56b4e22e09 sendemail[1988]: WARNING => Received:  454 4.7.1 <katanophoros@gmail.com>: Relay access denied
Nov 27 19:51:14 7e56b4e22e09 sendemail[1988]: ERROR => Exiting. No recipients were accepted for delivery by the mail server.

On the postfix logs I get are:
Nov 27 21:42:54 guest8 postfix/smtpd[14979]: NOQUEUE: reject: RCPT from unknown[172.18.0.5]: 454 4.7.1 <katanophoros@gmail.com>: Relay access denied; from=<no-reply@ellak.org> to=<katanophoros@gmail.com> proto=ESMTP helo=<7e56b4e22e09>
Nov 27 21:42:54 guest8 postfix/smtpd[14979]: lost connection after RCPT from unknown[172.18.0.5]
Nov 27 21:42:54 guest8 postfix/smtpd[14979]: disconnect from unknown[172.18.0.5]
Nov 27 21:46:14 guest8 postfix/anvil[14982]: statistics: max connection rate 2/60s for (smtp:172.18.0.5) at Nov 27 21:42:54
Nov 27 21:46:14 guest8 postfix/anvil[14982]: statistics: max connection count 1 for (smtp:172.18.0.5) at Nov 27 21:42:41
Nov 27 21:46:14 guest8 postfix/anvil[14982]: statistics: max cache size 1 at Nov 27 21:42:41

Do you have any sort of Idea how I will send emails inside docker containers?

A:

172.18.0.5
is not included in 
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 172.17.0.0/16
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 172.16.0.0/12
this includes all private 172.16 subnets.

