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The reason for this issue is that your mail client fails to authenticate to the server. Generally, this is caused by the MS Outlook SMTP authentication not switched on. Please refer to step 8 in our Outlook tutorial, in order to set your MS Outlook client to use the proper authentication:

Another possible reason for the issue is an Antivirus or Firewall software which is checking your outgoing mail connections. In case you have set properly your mail client as explained above and the issue persists, please turn off the antivirus and the firewall software you have installed on your local computer and try sending out e-mails again.

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