Users should keep an eye on their Junk Email folder(s) and mark items as "Not Junk" where appropriate. Users may also need to whitelist specific addresses or domains as needed. This can be done through Outlook Web Access (OWA), by adding addresses or domains to "Safe Senders and Recipients" under Options -> Mail -> Accounts -> Block or Allow. It is important to note that selecting the option "Don't move email to my Junk Email folder" does not turn off Junk Email filtering - it only disables a user's ability to Whitelist or Blacklist specific addresses or domains.
If you use OS X Mail (with client-side Junk filtering enabled) and are seeing a large increase in valid email being marked as Junk Mail, go to junk mail settings and disable "Trust junk mail headers in messages".