Since your traffic and routing equipment not to mention potential other resources being drained on your server are unique to your office the only way to get a real idea is to perform some benchmarks.
There are some straight forward tools included in Windows Server (Performance Monitoring) that you can setup, or you can purchase software for this.
I would start with Performance Monitor and set it to watch Network Bandwidth, Memory Usage, Hard Disk usage, and processor's. You will need to do this on a day that would be "slow" for traffic, medium and then on the 1st which you say is the most load. The first two measures are just to give you an idea of how your server is performing on the heavy day compared to a light day.
Example: Let's say on the 1st you send out 5 million emails all at once, the server ques them up and you see the hard drive speed max out, processor at 100% and memory fully used for the entire duration of the email sending. Now cut that back to say 4 million emails all at once and compare the results, this will help you determine the max amount of emails at one time your server can handle. You also need to take into account the results from the light day (say 5,000 emails sent and server load is 90% for 2 minutes) you can compare these to determine that at 3 million emails your load will be 100% for x minutes (approximately)
This is just benchmark your single email server, if you want to see how this effects the rest of your network you will need to monitor routers, users PC's, and perhaps even your ISP and your other internal resources.
Also check out the Exchange stress test tool from Microsoft (Exchange load generator in left column half way down)
http://technet.microsoft.com/en-us/exchange/bb330849.aspx
