Concept Question : Mail program with thousands of users

Question

I am working an internal mail system for our web access interface, message and TO user information is stored in a database, in other words not an email system. Our system includes thousands of users an a particular individual in a group may want to send a message to a few hundred of them at a time. I figured then that the user selection would need to be done on a separate page than the page that contains the text input for subject / body text of the message. Would it be a good idea to store all the selected usernames in a Session variable? My concern is that if the user changes their mind about sending a message and they skip off to another page, which could be a one of many, I would have all this data left in the Session. Is there a better way to do this? Is there a way to make sure that this Session data gets wiped? Am I being over concerned with the memory usage?