Blocked from sending emails (limitations)

I'm not sure why but I've been recently blocked from sending emails (two separate times now), presumably from going over the sending limits. I'm not sure why - I do have all my emails forwarded to my assistant, and send 5-10 emails a day, but between the two, I can't imagine that is going over 1,000 emails per day (I get maybe 50 inbound a day, so count 50 being forwarded to my assistant). If some one could help shed some light, it would be appreciated.