A hosted upload server, not a poll of yours: SFTP and SCP on port 22, explicit (AUTH TLS, port 21) and implicit (port 990) FTPS - cleartext FTP is refused. Usernames, passwords and SSH keys are managed in the console, with a per-service IP firewall and a browser uploader for people without a client. The login is upload-only: a file leaves the server the moment its upload completes, so nothing sits on it and there is nothing to poll.
DocEvent hosts an SFTP, SCP and FTPS server for you. Customers, partners and your own systems upload to it with their own login, and every completed upload enters the channel.