This error occurs when the SMTP server cannot authenticate successfully using the credentials provided.
Additionally, you may encounter this error when connecting with the Google SMTP server using an account that does not support "Less Secure Sign In Technology."
This error also occurs due to firewall and virus guard issues in rare cases.
Ensure that the credentials provided are correct. If you are connecting to a Google SMTP server, ensure that you've enabled "Less Secure Sign In Technology" and have disabled Two-Factor Authentication.
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