This document will show you how to check how much remaining space you have in Outlook.
Outlook - Prevent .ost File Errors
Outlook - Adding Room Calendars
Outlook - Configuring Out of Office Replies
Outlook - Creating Space
Outlook - Attachment and Recipient Limits
Outlook - Forward an Email as an Attachment
3. You can see how much space is remaining under Mailbox Usage.
3. You can see how much space is left under Mailbox Cleanup.