SMTP 5.7.3 Requested action aborted; user not authenticated. [SOLVED!!]

An e-commerce site that I maintain had been working fine, then all of a sudden emails stopped sending.
We started seeing this error:

Mailbox unavailable. The server response was: 5.7.3 Requested action aborted; user not authenticated


We hadn’t changed any code, and our username/password worked fine on the web. It turns out that our web host migrated our website to a new server (with a new IP address), and Microsoft’s security settings kicked in, preventing the emails from being sent.

The fix? Super easy.
Just go to your Outlook security settings and hit the “This was me” button. That let’s Microsoft know that it was actually you, and your emails will begin to fly!

Outlook SMTP

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