On the 'Advanced' tab change the outgoing server port to 587.
Many ISPs will block port 25, but all should allow port 587 for authenticated mail delivery.

If you need to, you could select the "Leave a copy of messages on the server" option. We request that you leave messages on the server for no more than 7 days. Our mailserver is not a long-term message store. This option is usually only used if you will be away and use the webmail interface to check your mail but still want to have it available for Outlook to retrieve when you return.