I have Send as permission on a mailbox. When I log on to my
mailbox and send an item as the other user, why does the item
appear in my Sent Items folder, not the other user's folder?
This behavior is a feature, not a bug. When you send an item,
you're the person who's sending it, even if you're doing so on
behalf of another user. This behavior is analogous to the way
real-world systems work: If I send a fax from my fax machine,
the recipient sees my fax number, even if the cover sheet says
that the fax is from the president of the United States.