where to find logs to fix problem not sending email

[Samo_slo]

Hi,

I've installed RoundCube Webmail 1.1.3 on Ubuntu 14.04 on my master server in a multiserver setup with ISPConfig 3 CP and the Roundcube plugin. My mail server is on a seperate secondary server. I can create email "accounts" domain + mailbox in ISPConfig and I can login to Roundcube with the created account.  The problem is that I can't send email, I just get the pop-up "A problem has occured! Message could not be send!" (that is translated becouse i get it in my language).
I would like to know in what logs i should look for the problems, do I have to enable logging? Maybe some one had the same problem and knows tho couse? 

[SKaero]

Have you configured SMTP?

[Samo_slo]

I've uncommented a few lines /etc/postfix/master.cf so it looks like this.

smtp      inet  n       -       -       -       -       smtpd
#smtp      inet  n       -       -       -       1       postscreen
#smtpd     pass  -       -       -       -       -       smtpd
#dnsblog   unix  -       -       -       -       0       dnsblog
#tlsproxy  unix  -       -       -       -       0       tlsproxy
submission inet n       -       -       -       -       smtpd
  -o syslog_name=postfix/submission
  -o smtpd_tls_security_level=encrypt
  -o smtpd_sasl_auth_enable=yes
#  -o smtpd_reject_unlisted_recipient=no
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
#  -o smtpd_helo_restrictions=$mua_helo_restrictions
#  -o smtpd_sender_restrictions=$mua_sender_restrictions
#  -o smtpd_recipient_restrictions=
#  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
#  -o milter_macro_daemon_name=ORIGINATING
smtps     inet  n       -       -       -       -       smtpd
  -o syslog_name=postfix/smtps
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes
#  -o smtpd_reject_unlisted_recipient=no
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
#  -o smtpd_helo_restrictions=$mua_helo_restrictions
#  -o smtpd_sender_restrictions=$mua_sender_restrictions
#  -o smtpd_recipient_restrictions=
#  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
#  -o milter_macro_daemon_name=ORIGINATING

And added the hostname of mail server to /opt/roundcube/config/config.inc.php $config['smtp_server'] = 'mail.myserver.ltd';  (not real hostname).
Do I have to configure anything else?