If you're coding your ASP mail system, sometimes you might watch as your SMTP mail starts out in the PICKUP folder and ends up in the DROP folder. Why does it do that?
In your SMTP mail area of IIS, you see the default domain entry. This is the process that handles all mail that comes into your SMTP system (i.e. mail that you send from ASP) that is not otherwise specified. This mail would be delivered based on the settings you make in the properties area of that default setting - including the name of the server that the mail claims to be from.
In addition to this default entry, you can also create an Alias Domain for any other domains you might own. Each one can have its own name. So where your default might say mail is coming from MySite.com, you can set up alias domains for YourSite.com and TheirSite.com.
Once you have those aliasses set up, each alias then sends its mail to the DROP folder associated with that alias. The mail will sit there in the DROP folder, waiting for further processing. It assumes that another system is going to poll that DROP folder and do something with the mail.
If you end up with mail messages in your DROP folder because you created an alias domain by accident, simply delete that alias and then stop and restart the mail server. Now drag all the messages from the DROP area back into the PICKUP area. They will process through the default queue, following your normal rules and get out to your recipients.