Flag users for re-verification after [xx] hours
This option helps maintain the user list by periodically asking the verification source if users still exist. After the designated number of hours, verified users are flagged to be re-verified the next time they send or receive email. Disabled users are not deleted.
Cache negative responses for [xx] minutes
When a query to a verification source shows that an account doesn't exist, this option will cache the result for the designated number of minutes. This helps limit the number of redundant queries made to the verification source.
See:
