appspot.com blacklisted?

[Gwyn Howell]

Have has some issues come in from users of my app hosted on appspot.com reporting email delivery failure with the following message:

Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 5.7.1 rejected content, black listed appogee-hub.appspot.com by dbl.spamhaus.org. #762 #895 

And here, it is reported as being on domain black list http://www.spamhaus.org/dbl/removal/record/appspot.com.

Our app includes a logo in our users email signatures. The logo is hosted on app engine. These users can not send emails at the moment, without removing the logo from the signature (or hosting it elsewhere).