Send emails on behalf of other accounts and save into sent items of on behalf of user account

hi, using ews api 1.1 in my asp.net application to send emails. Here I have a default account that sends emails on behalf of all other accounts. This avoids storing credentials of all users in the app database. But the problem is, sending emails on behalf of another account does not store a copy of the message in the sent items folder of the [on behalf of] user account.

So can anyone please help to find a way that stores the email in the sent items folder of the account the email is sent on behalf of?

Thanks..

-Jahedur Rahman