And that's at best, assuming you got no emails at all the previous day.
So eventually the maximum you'll get is sum 0 to inf [ 0.9^n ] ... Which is 10x
If allowance is fungible like you say, then sure, you're right. My experience of accountants and lawyers is that they don't treat these things as fungible, and are more likely to do something like:
Day 1:
allowance 200 emails
carried over 0
consumed 0
Day 2:
allowance 200
carried over 180
consumed 0
Day 3:
allowance 200
carried over 180 // 90% of 200 not 380
consumed 50 // consumption taken from allowance first
Day 4:
allowance 200
carried over 135 // 90% of 150 not 200 or 330
consumed 0
Maybe it's treated as fungible, but IMO the above is how lawyers and accountants would treat it.
So far the best email service I've found is Tutanota, who provide a custom domain for €12 / mo if you pay yearly [1], which is the cheapest I've found so far. I also like how my mail is encrypted at rest. The only downside is I have to use their web client which has limited features.