To clean our address list I would have to identify bad addresses and flag them in our address database so that I did not send e-mail to them anymore.  I decided that I did not want to delete the bad addresses, I just wanted to flag them as being bad.  But how do you determine that an address is bad?

Most SMTP servers will accept mail addressed to just about anyone in their domain, and only later figure out that the user does not exist. That means that whatever app you use to send mail will almost never know that there is a problem.  As far as your app is concerned, the SMTP server accepted the message -- period. 

I tried looking at so called "address verifier" components.  These components check the email address for syntactical errors and for non-existent domains, but they can not actually tell if the user part of the address is valid.  I used several of these to validate buggs.bunny@microsoft.com and was excited to find that Buggs does work at Microsoft these days,  but when I sent him an e-mail, it bounced back with the following message: "Delivery to the following recipients failed: buggs.bunny@microsoft.com".  The truth is that these "address verifier" components were no better at verifying addresses than my app was, so they were of no use to me.

So how do you reliably determine if an address is good?  The answer is -- you can't.  But you can determine if an address is bad when a message sent to it is returned undeliverable (bounced), and that is the key to solving this problem. 

The best part of this solution is that it is not dependant on extended SMTP features.  It will work all the time provided that the recipient's mail server correctly adheres to RFC-821, the minimum requirements for any SMTP server.  The SMTP protocol as outlined in RFC-821 provides for a notification mechanism when a message can not be delivered.  This notification mechanism works by creating a new e-mail message which is sent to the original sender to inform them that their message was not delivered.  This e-mail message is commonly referred to as a bounce.  The first step to cleaning our address list is to funnel the bounced messages into a central location where they can be programmatically analyzed.

The following 3 step process, will enable you to capture bounced messages, figure out which address in your database they belong to, and flag the record.