I have three mail accounts in Outlook 2007: 1x Exchange and 2x POP (Gmail and Yahoo). When I send an email from Gmail or Yahoo, I see the message in the Exchange outbox. What is it doing and how do I fix this?
More about :outlook multiple mail accounts sending mail exchange
It's because when you open Exchange, it sends a request to the Gmail/Yahoo servers (where your mail is really stored) for an update of any actions that have occurred since its last request (such as new e-mails/sent e-mails/drafts/etc). It includes activity from pretty much every mail folder.
So when you send an e-mail from Gmail/Yahoo itself, their servers save that sent e-mail. And then when you open Exchange, it syncs with Gmail/Yahoo servers - which is storing your inbox items, as well as your sent items - thus reflecting the outbound mail you wish to remove from Exchange.
To change this though...
If you have Outlook 2007 or previous:
Go to Tools > Rules and Alerts. From there select the E-mail Rules tab and uncheck "Move Sent Items from Web Mail".
If you have Outlook 2010:
Go to File > Manage Rules & Alerts. Do the same as above.