postfix mail server problems / usermin + roundcube

Hello
using a digitalocean lamp stack on ubuntu 14.04.2 + usermin-virtualmin +roundcube . have some vps online for some time working OK and now trying set up mail server. (my domains are registered to Goddady, etc)All set up is default on Postfix , Bind DNS,Dovecot, Roundcube.

i receive mail both on Usemin client and roundcube client

Problems :

  1. At USERMIN :
    i can sent mails to certain accounts like hotmail but not gmail by USERMIN - specif error at var/log/mail.log :
    Feb 10 08:27:46 webnode01 postfix/qmgr[22191]: 88991140150: from=info@poikilistoa.gr, size=702, nrcpt=1 (queue active)
    Feb 10 08:27:46 webnode01 postfix/smtp[23369]: 88991140150: to=gpanos@tekmon.net, relay=none, delay=0.05, delays=0.05/0/0/0, dsn=5.4.6, status=bounced (mail for tekmon.net loops back to myself)
    Feb 10 08:27:46 webnode01 postfix/cleanup[23368]: 97954146333: message-id=20160210062746.97954146333@webnode01.tekmon.net
    Feb 10 08:27:46 webnode01 postfix/qmgr[22191]: 97954146333: from=<>, size=2552, nrcpt=1 (queue active)
    Feb 10 08:27:46 webnode01 postfix/bounce[23380]: 88991140150: sender non-delivery notification: 97954146333

2)At ROUNDCUBE:
i cant sent mails at all - message i get: SMTP Error (535): Authentication failed.
specific error at var/log/mail.log :
Feb 10 08:00:41 webnode01 postfix/smtpd[22271]: warning: SASL authentication failure: cannot connect to saslauthd server: Permission denied
Feb 10 08:00:41 webnode01 postfix/smtpd[22271]: warning: localhost[127.0.0.1]: SASL LOGIN authentication failed: generic failure

Is it maybe something aditionally with MX records configuration at my DOMAINS side (Godday) ?
Thanks a lot
Any ideas?

I can only answer sending emails to Google/Gmail, Google expects that all emails will be signed/authenticated or it will either be sent to spam or rejected. Google also expects that a valid host name exist when validating the emails. Google also expect that a working RDNS exist for your main email sender host/domain.

I learned this issue with my own server after turning on my IPV6 IP addresses.