Unfortunately, the OnSend event occurs at a point in time before the final message ID of the outgoing message is available. This means you cannot easily identify the outgoing message’s message ID, which you would need in order to customize the Personal Subject. This problem has been present for many years, and is one we have asked Novell to address many times.
The only workaround is to collect key information about the message being sent (i.e. subject, date, etc), wait a short period, then locate the message in the sent items folder that matches the criteria. Once located, use the object API to update the PersonalSubject.
I hope this helps.