MaildirSpamDelivery

The goal is simple. Messages that are marked as spam but do not score high enough to get rejected: put them in the user defined SPAM directory.

Router section:

maildir_spam:
 driver = accept
 local_parts = !www:!root:!nobody:!postmaster:!abuse:!admin
 transport = maildir_spam_delivery
 condition = ${if def:h_X-Spam-Flag: {true}}

Keep in mind, though that certain tutorials do not use the X-Spam-Flag, but, rather, the X-Spam-Score, for those cases, use this router configuration:

maildir_spam:
 driver = accept
 local_parts = !www:!root:!nobody:!postmaster:!abuse:!admin
 transport = maildir_spam_delivery
 condition = ${if def:h_X-Spam-Score: {true}}

Transport section

Is just the same as your generic maildir delivery transport but the directory is SPAM

maildir_spam_delivery:
  driver = appendfile
  maildir_format = true
  directory = /var/mail/Maildirs/$local_part/.SPAM/  <---- example
 (...)
 [ rest of your settings here ]