Email from facebook.com or fbworkmail.com may be blocked due to SPF failures. This is primaruly due to Facebook's SPF records using redirects. To exclude these domains from SPF checks, edit the following file:
Add this to the file:
Domain_Whitelist = facebook.com, fbworkmail.com
You may add additional domains that you may be having problems with, but it is recommended that the root of the problem be addressed rather than bypassing the SPF checks.
Once complete, restart Postfix:
service postfix restart