You clicked the TEST ACCOUNT SETTINGS in OUTLOOK didn't you? This is a bug in OUTLOOK. The test message sends a test email with 'no Date header'. As the name suggests, this means a message without any date. Since the server doesn't accept mail with 'no Date header' (because it's required) the message is rejected. To test, send an actual message from OUTLOOK.
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If you want, you can try THUNDERBIRD. It's like OUTLOOK but made by a different company. It's completely free and works very well at home and at the office.