Hi @Ronald Cordina ,
According to my search, your HELO/EHLO name should be the system's fully-qualified domain name.
Here is an article for your reference: What exactly should HELO say?
(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
In addition, HELO/EHLO is more related with mail server and that the Outlook tag here we mainly focus on general issues about Outlook desktop client, it is recommended you reach out to your SMTP provider for assistance.
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