Porterhouse takes security very seriously. So seriously that ALMOST nothing is saved in the cloud. So what data do we save?
Porterhouse takes note of your contacts GUID. This is an ID that Apple generates that contains NO personally identifying information about that contact. This ID allows us to know which contact is connected to which message.
Because of this, if an account were to be compromised, the only thing they could read would be the message's content. That is it.
If you want to test this yourself, create another user on your Mac, download Porterhouse, and login. When you go to see your message recipients, nothing will be listed. You'll see a message stating "COULD NOT FIND RECIPIENT, CONTACT MAY HAVE BEEN DELETED.".
Again, this is because we save no information, if we can't find a matching GUID or (ID) in your contacts then as far as Porterhouse is concerned, it doesn't exist.