Message Store Error in Outlook
When a user tries to save an appointment on a shared calendar in Outlook she receives an error that said, “The message store has reached its maximum size. To reduce the amount of data in this message store, select some items that you no longer need, permanently (shift + del) delete them.”
This can happen if a Storage Quotas value has been set for Prohibit send and receive in Exchange.
To resolve the issue you can either remove items from mailbox that are no longer needed or increase the storage limit. However if you increase the storage limit it can take up to 2 hours for the setting to take effect. See this link for more details. A quick workaround (quick being relative to the size of the mailbox in question) is to move the mailbox to another database. Once the mailbox has been moved the updated storage quota setting will take effect.