> At the limit. Once a month's received allowance is used, further mail is refused with a permanent error and the sender is told;

Consider that you may be losing email without knowing even on the cheapest paid plan for $44 a year limits incoming emails to 500 a month. Most people don’t get 500 emails a month, but there can be months when this happens and the emails aren’t even in a “hold” until you upgrade. They’re just completely lost.

I’d rather have unlimited incoming emails than 15 GB or whatever higher storage is offered.