> 1. Attorney-client privilege.
I'm guessing most law firms use third party email servers, anti-virus, anti-spam and archive/audit systems which this would also apply to. It would also apply if you're using Raportive, Xobni or the like (or integrated time-tracking, billing, crm, etc.).
I can't speak for all law firms, but mine has been resistant to using any such third party services. I know some firms have relaxed their policies and there are plenty of lawyers who use Gmail, but the overall law here isn't settled and can vary from state to state.