This message is part of the RHP web site (therefore not malicious) and is alerting you to a failed email delivery, and suspension of further email delivery attempts.
What is happening is we suspend sending email immediately if we send an email and it gets bounced back because of an error on the recipients end.
We have to do everything possible to protect our email sending reputation. If we get flagged as spam - and emailing a non-existent email addresses is an indicator of this (think of a spammer buying an out of date database of email addresses) - then the whole service is crippled.
The message will give instructions on resetting this block though. (Email Settings)