I am facing the same issue not able to send the mail from Remote SASL Auth. My postconf -n is

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
command_directory = /usr/sbin
config_directory = /etc/postfix
daemon_directory = /usr/libexec/postfix
data_directory = /var/lib/postfix
debug_peer_level = 2
html_directory = no
inet_interfaces = localhost
inet_protocols = all
mail_owner = postfix
mailq_path = /usr/bin/mailq.postfix
manpage_directory = /usr/share/man
mydestination = $myhostname, localhost.$mydomain, localhost
mydomain = direction.biz
myorigin = $mydomain
newaliases_path = /usr/bin/newaliases.postfix
queue_directory = /var/spool/postfix
readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES
relay_domains = $mydestination
relayhost = [mail.otto4u.com]:587
sample_directory = /usr/share/doc/postfix-2.6.6/samples
sendmail_path = /usr/sbin/sendmail.postfix
setgid_group = postdrop
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =
smtp_sasl_type = cyrus
smtpd_sasl_path = smtpd
unknown_local_recipient_reject_code = 550

And facing the below error in Postfix Maillog.

May 17 09:32:34 oplq-test postfix/smtp[10151]: 77EA0150641: to=, relay=mail.otto4u.com[72.35.73.86]:587, delay=2.1, delays=0.05/0.02/1.5/0.52, dsn=5.0.0, status=bounced (host mail.otto4u.com[72.35.73.86] said: 554 qmail-dk: Cannot sign message due to invalid message syntax. (#5.3.0) (in reply to end of DATA command))
May 17 09:32:34 oplq-test postfix/cleanup[10149]: 98056150643: message-id=

Pls help me i had just stuck in this issue.

I have similar problem. For the beginning I added a "yahoo.com" to 'mydestination =" as you suggets. Still I cannot get email into my squirrel-mail account.

I can send out emails using squirrel-mail but I cannot receive.

the mail log error looks like

Mar 1 19:26:31 ns1 dovecot: Dovecot v1.2.15 starting up (core dumps disabled)
Mar 1 19:29:30 ns1 postfix/smtpd[31768]: connect from nm19-vm0.bullet.mail.bf1.yahoo.com[98.139.213.162]
Mar 1 19:29:31 ns1 postfix/smtpd[31768]: NOQUEUE: reject: RCPT from nm19-vm0.bullet.mail.bf1.yahoo.com[98.139.213.162]: 554 5.7.1 : Relay access denied; from= to= proto=SMTP helo=
Mar 1 19:29:31 ns1 postfix/smtpd[31768]: disconnect from nm19-vm0.bullet.mail.bf1.yahoo.com[98.139.213.162]

the postconf -n is

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
config_directory = /etc/postfix
home_mailbox = .mail/
inet_interfaces = all
mailbox_size_limit = 0
mydestination = datastore.ro., ns1, localhost.localdomain, localhost, yahoo.com.
myhostname = ns1
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
myorigin = /etc/mailname
readme_directory = no
recipient_delimiter = +
relayhost =
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
smtpd_recipient_restrictions = permit_mynetworks,permit_sasl_authenticated,reject_unauth_destination
smtpd_sasl_auth_enable = yes
smtpd_sasl_local_domain = yourdomain.com
smtpd_sasl_security_options = noanonymous
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes

Can you help me please?
Have a great day

I have a small concern, emails coming from my network are sent towards outside successfully
on the other hand those received from an external client (outside my network) have this problem (relay denied)

Excuse me, my English isn't will

it works after fixing it!
thanks a lot.