Your mailbox has exceeded one or more size limits set by your administrator.
Your mailbox size is 45307 KB.
Mailbox size limits:
You will receive a warning when your mailbox reaches 45000 KB.
You may not be able to send or receive new mail until you reduce your mailbox size. To make more space available, delete any items that you are no longer using or move them to your personal folder file (.pst). Items in all of your mailbox folders including the Deleted Items and Sent Items folders count against your size limit. You must empty the Deleted Items folder after deleting items or the space will not be freed.
See client Help for more information.
This means that your mailbox on the server is near the maximum allowed size. You should either delete unneeded messages or move some things to your local PC.