Outlook 2010 keeps prompting for password sending SMTP via SBS2008 Exchange

We have an SBS 2008 server.  My desktop PC had Outlook 2007.  I have one account that I connect to as a regular Exchange connection.  Another have a secondary e-mail address with a different domain name that comes to another account on the server.  I access it via POP/SMTP.  This allows me to choose which account to send outgoing mail from. This is necessary since Exchange/Outlook still only allow sending from a single domain.

I just upgraded to Outlook 2010.  The Exchange account works fine.  For the secondary address, I can receive via POP3 just fine.  I cannot, however, send mail through that account.  It just keeps re-prompting for the password.

I have a laptop on the network with the exact same server settings for the POP/SMTP account and it works.  On the Outgoing Server tab I've selected "My outgoing server (SMTP) requires authentication" and "Use same settings as my incoming mail server".  It makes no difference if I select "log on using" and type in the username and password.

I have verified the settings between my laptop and desktop for this account and they appear to be the same for this account.  The only difference is on the 1st account, which is the Exchange connection.  The desktop is direct connect on network whereas the laptop is set for RPC over HTTP.  But that should be unrelated since it's not the 2nd account that uses POP/SMTP.

What could I be missing?