Mail reaching sending limit without ever sending an email.

[Fcooper]

I keep running into this issue across multiple account on our email address through roundcube. "SMTP Error (550): Failed to add recipient "blank@gmail.com" (REJECTED - blank@companyname.com has reached the mails per hour limit).

I can send emails at the top of the hour and can reach customers. But I can send them in the middle of the hour without getting that message. Is there anyway for this to stop?  Did someone highjack our email system to send spam? Its extremely frustrating any help would be great!

[SKaero]

That error is returned by the mail server not Roundcube, ask your email provider what you can do to prevent the error from occurring.