> NOQUEUE: reject: RCPT from forward20.mail.yandex.net[95.108.253.145]: 454 4.7.1 ........

Вам надо сделать следующее: ........

Затем перазапустить postfix и всё сразу заработает!

# 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 (Ubuntu)
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/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
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 = sd1234-02.keymachine.de
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = mail.site.ru, sd1234-02.keymachine.de, localhost.keymachine.de, localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all

luser_relay = login
local_recipient_maps =
virtual_alias_maps = hash:/etc/postfix/virtual_maps

> telnet localhost 25
> Trying ::1...
> Connected to localhost.
> Escape character is '^]'.
> 220 sd1234-02.keymachine.de ESMTP Postfix (Ubuntu)
> 421 4.4.2 sd1234-02.keymachine.de Error: timeout exceeded
> Connection closed by foreign host.

Ну и тестируем-отправляем (в чем проблемы - то),
приблизительный порядок комманд, без helo/ehlo:

rcpt to: <touser@mycool.domain>
<= PRESS ENTER KEY NOT ANY KEY
here are server reply: must 'ok'

from: <fromuser@mycool.domain>
<= PRESS ENTER KEY NOT ANY KEY
here are server reply: must 'ok'

data
<= PRESS ENTER KEY NOT ANY KEY
here are server reply: must 'ok'

Subject: TTesTT
<= PRESS ENTER KEY NOT ANY KEY
TTesTT
<= PRESS ENTER KEY NOT ANY KEY
.
<= PRESS ENTER KEY NOT ANY KEY
<= PRESS ENTER KEY NOT ANY KEY
here are server reply: must 'ok'

P.S.
всего делов-то... и внимательно прочитать базовые мануалы,
поверьте это не с SIP`om возиться - любая ошибка в настройке любого MTA диагностируется по 5 (максимум) строчкам из /var/log/mail* и/или telnet`om на соответствующий порт(ы)