error on smtp my Prestashop

[floriankajbe]

Error: Please check your settings
Expected response code 354 but received code "503", with message "503-All RCPT commands were rejected with this error: 503-Invalid authentication (you can only send from @smtpusername or its aliases) 503 Valid RCPT command must precede DATA"

My current email address and its alias addresses are correctly entered in the settings. However, I'm having trouble authenticating and sending emails from my domain "@smtpusername ".

Can you please help me and give me a solution or guide how to fix this error? I am grateful for any help you can offer me.

Thanks in advance.