Error message - what does it mean?

unfortunately, it can refer to a variety of possibilities as this URL explains: https://mailflowauthority.com/troubleshooting/550-5-7-1-rejection There are links there that may help. If this is web-hosted email address, I suggest reviewing with your web host provider, as it may be that security in the server is lacking - something you cannot address as a user. It may be something simple, such as incorrect SMTP server setup, or possibly you have multiple accounts with that email provider and have inadvertently set one of the accounts to use the SMTP setting for the other. Posting screenshots of server settings pane and of SMTP server settings pane may help, but that, in itself, doesn't cover all the possibilities. I share this info, but I am not one who can dig deeply into this.