Users occasionally report that when they file an email they get a message saying that their email was not filed and they received a message to refile it after fixing this problem:
“Error: field integrity exception: WhoId, WhatId……)
This means that an attempt was made to file an email to a Lead and an Account. This is not a valid combination, since Leads are associated with Companies and Contacts are associated with Accounts.
The resolution is to delete the account name or delete/replace the lead name from the filing criteria before filing the email.