How many mass emails can we send before we are blacklisted?

Our company is wanting to switch from sending bid invitations via fax to email.  During the busiest months we will probably send 5,000 to 10,000 emails along with the our normal company email.  Each bid invitation will be sent to between 100 and 400 email addresses depending on the size of the project.  Our company has about 15 employees who have email addresses, therefore I would estimate our current monthly email volume is around 20,000 emails per month (15 user x 70 daily emails x 20 business days in month).

I spoke with our IT provider and they warned me that if we send too many emails we could become blacklisted.  

Most of the emails are going to corporations with corporate email addresses, not yahoo, gmail, hotmail etc.  Would this kind of volume increase cause us to get blacklisted?  

Thanks,
jacob