IMAPsync synchronizes imap folders, not incoming mail. Strictly seen, these bypass the built in mail system.
I did suggest a solution a while back using an independant VM or Container to provide imapsync features, and to use postfix as a local MTA to forward the mails to the real smtp incoming queue of NS8 mail. That works, but it was a concrete idea for a user with the same issue as you have.
It was also to provide an idea for the dev team, how a solution could look like.
Not ideal, but a working workaround and also step toward a solution…
