require ["envelope", "fileinto"];
if envelope :is "from" "test@mydomain.com" {
    fileinto "Test";
}

dovecot: lda(test@mydomain.com): Error: sieve: msgid=<[...]>: failed to store into mailbox 'Test': Mailbox doesn't exist: Test
dovecot: lda(test@mydomain.com): sieve: msgid=<[...]>: stored mail into mailbox 'INBOX'

namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    auto = subscribe
    special_use = Drafts
  }
  mailbox Junk {
    auto = subscribe
    special_use = Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = Sent
  }
  mailbox Test {
    auto = subscribe
  }
  mailbox Trash {
    auto = subscribe
    special_use = Trash
  }
  prefix =
}