When does message get to Sent Items folder

Hi,

I'm writing transport agent for Exchange server, which processes message attachments. As attachments are changed I need to update Sent Items folder. I do it using EWS.

Issue: sometimes message is not found in Sent Items.

So my question is: Who and when saves message to Sent Items folder? Is it some part of exchange server? Or is it Outlook who does it?

Can someone point me to useful articles or give a hint on where to search?

Thanks,
Oleksii