Hi There,
C3x0D was designed as an outbound only box, the idea being that your internal servers would send mast amounts of mail to the C3x0D and we process and deliver.
The main differences are:

1: Multiple IP Interfaces
Deliver from 256 source addresses to minimise rate limiting issues
2: Configurable Source IP per destination
Nominate specific source IP for certain destinations
3: Virtual Gateways
10,000 Concurrent connections, delivery queue per destination
4: Domain Key signatures
Validate you are the trusted source of the mail
5: Destination Controls
Control delivery rates to suit ISP acceptance policies
6: Configurable Bounce profiles per destination
Customize retries and delay periods for differing destinations
7: Message and Content Filters
Apply disclaimers, unsubscribe information and tracking headers
8: Optional Sophos engine
Ensure clean mail only is sent

More information on the support portal regarding C350D, or messenger me and I will send you the pdf.

Laters :)