If you're getting that error from MS Outlook, then you'll have to find out from somene familiar with MS Outlook what it means.
Often ISPs block their outgoing port 25 traffic from leaving their network. Does Outlook send mail if you use your ISPs mailserver settings for outgoing mail? If so, then I recommend using it, or at least checking with your ISP to find out if they block outgoing port 25 traffic before spending time checking your server.
If your ISP does block port 25 traffice you might try connecting for outgoing traffic on port 587 instead of port 25; modern versions of the exim.conf file will also accept connections from authorized senders on port 587.
Jeff