The folders "inbox" and "sent messages" are each limited to 1000 messages.
As soon as ONE of both folders consists of more than 950 messages, you will be informed; When sending a new message, you will be asked to delete some other messages first.
As soon as you have more than 1000 messages in your inbox or in the folder "sent messages", you will not be able to send messages and you will have to empty them first. So delete unnecessary mails or move some mails to private folders.
Please note that the total number of all messages in all folders is limited to 2000.