All files and information sent with the Pro or Business accounts are transferred using HTTPS - encrypted using an AES-256 key.
Our servers are behind 2 sets of firewalls, and access to the data directly is maintained by windows access after that. Filenames are encrypted on disk, and the amount of data that resides on the servers makes it nearly impossible to recover and decipher information from the hard drives.
On our website, you can theoretically access all downloads, provided you guess the ID of the upload.
This ID is determined randomly and there is 26^15 possibilities (or 1 677 259 342 285 725 925 376 combinations if you will). In addition, you can add a password to all uploads for extra security.
Filemail is considered very secure - and is being used by numerous companies and organizations to send sensitive data. We control our own servers - avoiding the prying eyes of the NSA and others.
You can read more about our security here.