I would like deliver not to automatically create subfolders for mails
addressed to user+some_strange_name@domain.com, but nevertheless
automatically create INBOX for new users. I'm not sure whether a new
option like -M or -m -m would be appropriate, i.e., whether anyone
needs the current -m behaviour.

It looks to me like one would need to change
mailbox_open_or_create_synced()

     if (box != NULL || no_mailbox_autocreate)
             return box;

to something like (assuming an enum mailbox_autocreate {_never,_inbox,_always}

     if (box != NULL)
             return box;
if (mailbox_autocreate == mailbox_autocreate_never || strcasecmp 

(mailbox, "INBOX") != 0) return NULL;