We use procmail to reformat and filter email messages sent from many lead capture websites where the clients enter their reply email address manually.
When we receive the message at our application we send a welcome email, and receive back the Undeliverable Email message where the clients entries are invalid, but I would like to be able to kill messages with undeliverable email addresses on our mail server before they are delivered to our application.
I have found discussions on filtering masqueraded addresses, but since the IP of the sending system is not related to the entered client's email address, this does not appear to help. I would think that it should be able to query the email server to identify if the address is valid. If anyone has any ideas on how I might be able to accomplish this, the information would be greatly appreciated.
You cannot reliably query a remote mail server to check validity of a delivery address.
You can create a unique sender address for each delivery address. Then if delivery fails the message will bounce back to your server, where you can process the bounce message and mark the delivery address invalid.