SMTP error 538 occurs when the authentication mechanism requires encryption to complete a client's authentication process, and without the encryption, the authentication process fails. It is primarily due to the mail server or email providers requiring encryption to add a layer of security to the authentication process.
To resolve this error, the client must contain the required encryption to authenticate with the email server successfully. These settings may include TLS versions and encryption algorithms.
The user must obtain these encryption requirements from the email server administrator/email service provider.
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