sending mails from an additional mailbox

Hi,

I'm working on a user outlook account and they need to be able to send as a different user

I  have added the additional mailbox within outlook for the second user on a domain.  The default user has all the necessary rights to send as etc. and the mails are sending out fine.

The issue I have is that when I send a mail as the second user then the mail will show up in the sent items of the default users mailbox.  How can I get the mail to stay in the additional mailbox sent items when sending a mail as the additional user.

Thanks in advance
Paul