There are a couple of messages worth noting in your logs:
Nov 30 08:52:38 25612-21212 postfix/master: fatal: bind 0.0.0.0 port 25: Address already in use
It looks from the subsequent entries in your logs that Postfix eventually started, but at some point in the past, another service was bound to port 25. Did you install fetchmail
What's the output of the following command?
And how about
As for these entries
postfix/smtpd: warning: 126.96.36.199: hostname 207-13-31-64.servebyte.net verification failed: Name or service not known
These are inconsequential unless that is your
IP/hostname listed. Is your IP 188.8.131.52? These messages indicate that the hostname and IP address don't match. Normally, these messages are the result of bots probing the server with unusual network configurations (IP spoofing, etc.) and can be ignored.
Finally, these messages
postfix/smtpd: warning: unknown[184.108.40.206]: SASL LOGIN authentication failed: authentication failure
seem to be legitimate authentication failures... perhaps you missed a step in the tutorial and didn't install a required library somewhere along the way. We'll investigate that after you've answered my other questions.
As with squirrelmail, roundcube accepts none of the login credentials of email addresses created in ISPConfig...?
It bears mention that for a mail user to be able to log into webmail (Roundcube or Squirrelmail), his mailbox must
have IMAP support enabled. If this is already the case, then there's a problem with your IMAP authentication.