Sorry for the English but I write faster in it...
Go to Settings, click on View Account Parameters. After a short while it will present you with the parameters of your mailbox including:
1) Maximum possible mailbox size in KB
2) Current mailbox size in KB (calculates messages only, not contacts, appointments or notes)
This gives you an idea of what the system thinks of your usage. Your maximum possible mailbox size should be 20000KB. If it is less, you should let Customer Care know.
Hope this helps...