Enable Exchange 2013 Spam Filtering

MX Logic or Postini:

If you use MX Logic, Postini, or a similar spam filtering service, you want to make sure you white list the IP ranges the service uses to submit email to your server.

Set-TransportConfig -InternalSMTPServers @{Add="208.65.144.0/21","208.81.64.0/21"}

These IP ranges are for MX Logic.

Enable RBL Checking and Sender ID Config:

If you are not using a 3rd party filtering service it is a good idea to enable RBL checking, and delete spoofed email as determined by SenderID.

Add-IPBlockListProvider -name bl.spamcop.net -lookupdomain bl.spamcop.net
Add-IPBlockListProvider -name zen.spamhaus.org -lookupdomain zen.spamhaus.org
Set-SenderIDConfig -SpoofedDomainAction Delete