IMAP:
You are right! ‘schloegl’ can log in and see what’s inside. ‘office@schloegl-software.at’ could not, because this is an alias email address for user schloegl only. This is what Oulook does, if you set up an email accout (user=email) and further more if you check the settings it will not report an error.
Anyway IMAP works! Thanks a lot 
but
POP3:
Same as before and I do log in with correct user ‘schloegl’ there. Would like to add, that same clients were working with NS 6.8 final.
here the logfile created by using Thunderbird:
Jan 31 14:03:34 dc2 dovecot: pop3-login: Login: user=<schloegl>, method=PLAIN, rip=10.140.1.84, lip=10.140.1.6, mpid=14875, TLS, session=
Jan 31 14:03:34 dc2 dovecot: pop3(schloegl): Disconnected: Logged out top=0/0, retr=0/0, del=0/0, size=0
here the logfile created by using Outlook2016:
Jan 31 14:07:15 dc2 dovecot: pop3-login: Login: user=<schloegl>, method=PLAIN, rip=10.140.1.2, lip=10.140.1.6, mpid=15095, TLS, session=
Jan 31 14:07:15 dc2 dovecot: pop3(schloegl): Disconnected: Logged out top=0/0, retr=0/0, del=0/0, size=0