How To Use The Verify Peer Feature With Several Common MTAs

The verify peer feature is checking for the existence of a user on a designated mail server. Most of the time, those servers are not configured to respond with an error when the client party issues RCPT TO command for a non-existent user – with the purpose of preventing dictionary attacks. The most common way of dealing with such a situation is to accept any recipient and only afterwards generate a bounce message that the initial email has been destined to a user unknown to the mail system. This document will guide you in configuring several popular MTAs to response – during the SMTP transaction – in a way that the verify peer feature of Syneto’s Mail Proxy will find useful in checking for the availability of a user.

Postfix 1.x

Edit main.cf and add the following line:
local_recipient_maps = proxy:unix:passwd.byname $alias_maps

Postfix 2.x

By default Postifx 2.x rejects unknown mailboxes.

Exim

Qmail

Microsoft Exchange 2003

1. Start the Exchange System Manager (Start -> All Programs -> Microsoft Exchange -> System Manager) 2. Expand the Global Setting option and right click on Message Delivery; select Properties from the menu 3. In the Message Delivery Properties windows, select the Recipient Filtering tab 4. Check the Filter recipients who are not in Directory box

4. Click OK/Apply until clear 6. Still in Exchange System Manager, expand Servers->Protocols->SMTP; right click on Default SMTP Virtual Server and selectProperties from the menu 7. In the Default SMTP Virtual Server Properties window, in the first tab (General) click on the Advanced button near the IP address selection

8. Select, in the new window, [All Unassigned] from the table and click Edit

9. Enable Apply Recipient Filter

10. Click OK/Apply until clear

Lotus Domino Server 7

1. Start IBM Domino Administrator
2. Select the domain you wish to administer
3. Click on the Configuration tab
4. From the left side pane expand the Messaging label and select the Messaging Settings
5. Select the Messaging Settings tab (right pane) then Restrictions and Controls then SMTP Inbound Controls
6. Set Inbound Intended Recipients Controls as is presented below:

7. Apply configuration and save