I haven't heard any news on this front in quite a few years. I seem to recall that the 400/hr limit was the grand total of incoming and outgoing mails, and that 300/hr for outgoing was a safer limit. I think there was a burst limit of 20 per minute. I think that if you set up with those two in mind (i.e., 5 per minute), you should be OK.
If you're still concerned that you will be penalized, you could always open a support ticket and ask for an official declaration.