We've already sent out a lot of beta invitations and received great feedback so far. Since we needed to address a few issues like better Mavericks compatibility, improved Gmail integration and optimized IMAP syncing, we decided to postpone sending out new invitations until we've finished the next beta version. Once we've completed our internal testing we'll give access to this beta to all people that have registered. If all goes well this will be during the first week of September. Thanks for your patience!