Nah greek is something completely different ;-)
Anyway .. since it's a 15min tail, there's no mail arriving at your system.
As of what i understand, you want clients to be able to manage their domains in the web admin of ISPConfig, well that's possible BUT then you have to make that server authoritative for those domains. That means that your ISPConfig server is the one who is managing that domain and what that server contains is the correct setting.
f.e. if you buy domain abc.com (ip: 18.104.22.168) and run it as authoritative on your server i can add abc.com on my server as well but with another ip 22.214.171.124 .. though when someone asks the ip for abc.com it will eventually ask YOUR dns server what the ip is, it won't ask mine, since you're authoritative.
For now, GoDaddy is the authoritative server for your domains, so all requests for your domains will go to GoDaddy, not your ISPConfig server.
I'm running multiple ispconfig3 servers that do mail only, i've 0 dns entries on those servers. I manage all domains and settings for it (mx,cname,txt,a,aaaa) on the nameservers.
at GoDaddy say:
example.com MX mail.example.com
mail.example.com A <your public ip>
in ispconfig you only have to create the mail domains and a mailbox .. no dns zones needed. ispconfig only needs to know which email domains are hosted on itself, nothing more.
Now if i send a mail to email@example.com
my server will check "who's authoritative?" .. godaddy .. It will get <your public ip> from godaddy and will contact the server at <your public ip> .. my server will say "yo, i got an email for firstname.lastname@example.org
!" your server will check the email domain list for "example.com" and then if there's a username "user" attached to it .. if so, accept the mail et voila.
So if you want all dns configuration to be done on the ispconfig server, make it authoritative, if that's possible at godaddy i don't know.