I’m baffled by that I haven’t seen this in the forums.

The 550 SMTP error = ‘no such recipient here,’ I don’t know where the Please stop spamming come from, perhaps your ISP is on a spam black list.
Did you ask for an email receipt (http://cr.yp.to/smtp/mail.html), this may have triggered this ?
Do you have another email address you can try with a different domain name ?