I attempted to add a mailhost to my profile, however, SpamCop reported that the process failed because the MX host for the destination domain rejected the message. The MX server returned a 4xx error code and an error message which indicated that the message was rejected due to greylisting and should be retried later. The fact that the SMTP error code and message text were returned to the web application in real time leads me to doubt that the message will be queued and retried. Essentially, this means that a SpamCop user cannot add a mailhost which is configured to use greylisting. I find that somewhat ironic, given the fact that the SpamCop email service uses greylisting.