This approach has been followed by later languages such as maildrop and sieve. The maildrop MDA also includes filtering functionality. The structured filtering language is robust enough to allow maildrop to work with virtual mailboxes. If the input to maildrop is a file then a temporary file is not used. When the client exits the session, mail marked for deletion is removed from the maildrop. On Unix-like systems, procmail and maildrop are the most popular MDAs.