I have the same problem. Error code 553 every time I try and send emails, even one liners, either by Mac Mail or Webmail. No problem receiving emails, just sending.
Hi @JonathanHarper, welcome to the forum, and sorry you're getting this error when trying to send emails.
I've sent you a Private Message so you can get in touch with the Mod team and we'll be happy to help you with this.