Emails remaining in outbox

All,

I am using the Outlook 2007 trial and am really liking it so far. I do have
one hitch though.

I use Outlook 2007 to connect to both my company Exchange server for
corporate email as well as using it to connect to a mail/smtp server for
personal email. One nice feature is that when you reply to a message or
create a new message, there is a drop down menu allowing you to choose which
account you want to send your message from. When I use Exchange there are no
problems. However, when I select one of my pesonal email accounts and click
send, the email remains in my outbox indefinately even though the recipient
has received the email.

I should note that I receive emails fine from both the Exchange and Mail
servers. I only have this problem when I send using one of the Mail servers.

Any help would be appreciated. Thanks.

Abraham