If the user have a lot of emails on the server, it would impact Inode count: one email = one file on the server, and one file = one Inode.
I often put in emails as an example as it is something most are not aware being stored as files on the server.
The only safe way to delete is to use the webmail, or have a client setup with IMAP (deleting in client will delete on server), or using a client with POP3 and never save the emails on the server, and only have them locally on the client.
That being said, emails is only one thing that would contribute to Inodes, many small cache files (some cms do this) will add them faster, some cms use up a lot files even with just a fresh install and no content (Drupal for instance used up somewhere around 20 000 Inodes fresh basic install if I recall right from my last check), so not all things will be suitable to have on free hosting for more then testing purposes.
