Recipient Validation fails for valid recipient

Question

I have entered a single Mail Delivery Server (192.168.1.18) and a single Mail Domain (ex.com) set to use that server. There is one valid user, me@ex.com. 
 If I telnet to 192.168.1.18:25, I am able to manually submit an email to me@ex.com without error. 
 
 When I disable recipient verification and try to send an email through the appliance, the rejection message is: 
 "me@ex.com: Recipient address rejected: User unknown in local recipient table." 
 
 When I set recipient verification to downstream SMTP look-ahead, the rejection message is: 
 "me@ex.com: Recipient address rejected: undeliverable address: unknown user: 'me'" 
 
 I setup a packet capture on the server at 192.168.1.18, and no traffic is sent from the email appliance(hardware) to that server with either verification setting. 
 
 Obviously I'm missing something, but I'm at a loss for what it is. Can anyone offer a hand?

Red_Warrior · Answer

Hi Mike, 
 Via downstream SMTP look-ahead (recommended): The Email Appliance's mail transfer agent (MTA ) uses SMTP recipient validation. The MTA will connect to the internal mail server to confirm that an address exists. 
 I would only use this if your not using AD 
 mta connects, post fix responds with .. wait a second... postfix sends a telnet request downstream to exchange and sends an envelope from.. Exchange answers .. Valid. telnet session ends, appliance accepts the message. the message is processed and delivered down stream. Once Exchange actually gets and end of DATA command exchange validates the address and rejects it. This generates a NDR that is delivered back tot he appliance and the appliance in turn trys to send it out for the next 5 days. 
 Exchange does not actually validate an address until that final . in the DATA command. 
 
 Via Configuration Sync: You can maintain lists of some configuration data, including recipient validation, in text files. With Configuration Synchronization you can use the SCP protocol to sync those lists to the Email Appliance. 
 not to many people use this, but it will allow you to manually create and upload your own lists to the appliance. 
 
 Via directory services: The Email Appliance's mail transfer agent (MTA) uses directory services queries to determine if messages are addressed to valid recipients. You should ensure that your directory services settings are configured correctly. 
 by far the best option if you have AD integration. postfix will query the list immediately when the mta connects and accept or reject the message before it is accepted. This also prevents exchange from generating NDR's down stream. 
 
 Disable recipient validation: Turns off recipient validation. It is recommended that you do not disable recipient validation, unless you have specific requirements. 
 Yes, um don't disable validation.. 
 
 If you wish to test the connection here is a sample email via telnet. 
 
 Telnet 223.212.251.21 25 Helo localhost Mail from: support@sophos.com Rcpt to: test@test.com DATA Subject: test from email appliance From: santa@thenorthpole.com ENTER MESSAGE (blank line) .