The SMTP "Error 503 Valid RCPT command must precede DATA" can occur when sending an email if the email client did not authenticate with the email server. Therefore could be due to issues with the SMTP authentication settings on the SMTP client.
It is essential to ensure that the SMTP client's authentication settings do not have errors and validate the user credentials.
Setting up SMTP authentication on the email client differs with each client; you may refer to the configuration guide for your preferred email client for a detailed step-by-step guide.
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