All built-in Junk/Spam filtering by your email clients (Outlook, Windows Mail, Mac OS X Mail) are NOT RELIABLE. They cause false alarm, marking legitimate email as Spam and throw them to the Junk folder.
To add, many antivirus softwares intergrate with your email clients such as Outlook to provide additional protection. THIS IS BAD TOO! This unseen layer of protection causes missing email and attachment.
To turn off Junk filters in your email client:
1) Outlook and Windows Mail > Junk Email Options. Choose No automatic filtering.
2) Mac OS X Mail > Preferences > Junk Mail > Disable
To turn off third party virus integration with Outlook
1) Symantec Endpoint Protection > Change Settings > Antivirusâ€¦ > Disable Outlook Auto-Protection