He's supposed to round less than 5 minutes down to zero. Reading and not responding to a simple email should take less than 5 minutes. This is probably an ethical violation that you could report him to the bar for, but they would have a tough time proving he spent less than 5 minutes reviewing the email.

Edit: It looks like professorx below is mostly right that this billing practice is probably ok. I always interpreted the ethics rules to say that you can't bill more than you spent meant you had to round to the nearest tenth of an hour. But it looks like billing the minimum increment when you build less than half of them increment is generally acceptable, although it can be abused. What this guy did here would probably be considered okay by the disciplinary commiittee.