SMTP Error: 5.7.57 SMTP / Client was not authenticated to send anonymous mail

There are multiple possible reasons for this error

Reason 1
Missing or incorrect Login info

Solution 1
SMTP Settings check the 'Use Authentication' and Provide Credentials for the SMTP (Contact your SMTP Provider if you do not know what they are)

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Reason 2
TLS not checked

Solution 2
In SMTP Settings check the 'Use TLS'

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Reason 3
From Email missing or not an email address

Solution 3
Check that you have entered a valid from Email in SMTP Settings