Hello,
The ESA do not have a email validation file like you have on a Qmail server.
You have different options with the Appliance to validate the recipients without the need to use LDAP.
You can for example use the SMTP Call-Ahead feature which will check the recipient email address for new incoming messages with your backend SMTP Server and if the internal SMTP Server accept them, the Appliance will do the same.
Further you can also use the Recipient Address Table (RAT) and insert each single recipient email address instead of your complete domain. With this the Appliance will only allow the recipient email address which are part of the RAT. But a long list of email addresses within the RAT are not a good option as this will need more performance from the Appliance to check each recipient email address against the Recipient Address Table.
Accepting a message first and then rejecting them using the Mail Policies / Content Filters are not a good option, as this could cause that messages send with a fake sender email address get bounced back to a different remote smtp server and so it could happen that your IP addresses get on Blacklists.
The best is if you take a look on the SMTP Call-Ahead feature of the ESA. This feature can be used to do a recipient validation with the internal smtp server without the need to use LDAP and so the Appliance only accept valid recipients and reject all others.
Best regards,
Mark.
