I haven't reached my limits and I cannot open any emails, why?

Limits do not reset at midnight, they reset 24-hours after you reach the limit. As that happens at your convenience, that time will always change.

I'll provide an example on how the limits work by using the limit of 10 emails. Let's say that on Monday you only sent 5 emails. Now when the next time you login will only have 5 emails left. Keep in mind that opening an email from someone whose unlocked pictures for you will count as 1 email.