Subject Auto Number and Send Event

Hello

Need some help to do the following, i need to use a button to set the default subject for a new message this default subject it's saved somewhere (xml database) and uses a number to know the next message number, and then detect if the mail was "sent" (user pressed the send button) to update the last value of the number.

For example i have a little xml database with this:
SubjectTitle | Style | LastNumber | Year
MySub1 | Mickey PT {Year} - {Number} | 4 | 2011

When the user uses the button to create the new message it fills the subject with this: Mickey PT 2011 - 5 (increment the number)
Then i need to check if the user really "sent" the message by trapping the send button click, to update the value of 4 to 5.

Thanks