Is there any way to implement the same bandwidth management in (ISPconfig3) Dovecot as there is in Apache?
You can limit the number of emails and most likely the traffic as well with policyd for postfix, so you can limit in and outgoing emails, if this is what you want. There is a ispconfig policyd integration planned, but at the moment you would have to use the policd standalone web interface if you like to use it in ispconfig.
Actually the problem now seems to be Outlooks constant IMAP "sync" checks. No SMTP traffic problems so far. Some clients Outlooks sync all the time. Non stop.