I want to be able to send a email using a secondary SMTP address. I found a way to do this sometime ago, but can't find my notes on it!

You can't do it natively but there are some third party tools that will facilitate it. None of them are free however.

Setup a POP/SMTP account with the secondary e-mail address and use that to send e-mail. You can use the Exchange server as the SMTP server and don't bother configuring the POP account part. Then tell Outlook to only send using that account(don't receive) via the "Define Send/Receive Groups" options. Works well and is really easy to use.

That is a really good suggestion. A bit kludgy, but the only way I could figure out how to do it without having other mailboxes or buying a 3rd party product. This is a pretty common request. Microsoft should take note of this and hopefully modify Outlook so that it would allow this in the future.