[...]
plugin {
  quota = maildir:User quota
  quota_rule = *:storage=10G
  quota_rule2 = Trash:storage=+100M

  # LDA/LMTP allows saving the last mail to bring user from under quota to
  # over quota, if the quota doesn't grow too high. Default is to allow as
  # long as quota will stay under 10% above the limit. Also allowed e.g. 10M.
  quota_grace = 100M

[...]
}

[...]