I've installed MailWasher Server, but it does not filter inbound email

There are some possible reasons for this.

But first, perform a couple of quick tests.

Perform these ON the machine MWES is installed on:
1) telnet to the MWES proxy port: telnet [external IP address] [port] - should result in a HELO string with (MP) on the end.
2) telnet to the MTA port: telnet [IP address] [port] - should result in the above, minus the (MP)

Perform the same test from a PC outside the network.

Make sure all firewalls have allowances for MWES. - this is rather important, as windows auto provisions firewall rules, and removes them when you disable a MTA service such as Exchange.

1. The MTA has not been moved to port 26

Change the listening SMTP port from 25 to 26 for your MTA

2. No domains defined

You will need to add any domains used at Settings >> Setup >> Domains, to prevent your network appearing as an open relay.

3. Router configured to present an internal IP address

When an external router or connection to the internet does not translate the external address properly, any incoming mail seems to be presented from an internal LAN address.

This will prevent any spam checks from being performed as the incoming mail is deemed to be from an internal source. ie, Services like RBL's and Greylisting won't work because they rely on checking external IP addresses.

Please reconfigure your router so it translates the external IP addresses.

4. Your hard drive is full.

5. If you go to Settings>>License and see License is unknown, please email mwes-support@firetrust.com

6. Your setting for Proxy Hostname in Settings>>Setup>>System Configuration, is set to a local IP address. 

MWES lets email through if it’s local, and since you have MWES on a local IP then all email will pass through.

You can change this by adding this registry key as a string in
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Firetrust
use_is_routable and give it the value= 0
Then restart the Mailwasher Service.

But RBL’s and greylisting won’t work as they require an external IP address.