Comment by Sauerlaender

There are no high scalability improvements, it is a regular Dovecot Pro setup.

44.000 mailboxes is not really big from Dovecot Pro point of view, there are dozens of other service provider customers running six to eight digit mailbox numbers on it. Biggest European installation serves 40M mailboxes although these are consumer / SMB mailboxes which lower load provide and size compared to Schleswig-Holstein.