This is generally an issue with Outlook not properly authenticating to the server before attempting to relay mail. In Outlook, select the email account profile used to send and receive mail. On the outgoing mail server tab, tick the box for "my outgoing mail server requires authentication". The "use the same login as the incoming server" option will also enable by default when that is done; leave that setting in place. On the Advanced tab, select TLS from the dropdown box next to "use the following type of encrypted connection". Save the settings, and Outlook should be able to send without issue.
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