Saving draft to Drafts Folder

I'm using a shared mailbox with 7 other people and we save many emails in our "Drafts" folder for peer reviewing. We are all using Outlook 2010 clients on Window 7. Now I'm trying to save an email to the "Drafts" folder but I'm getting an error message. The email has 4 attachments adding up to just under 400 KB. Now the OST file for the shared mailbox is only 1 GB is size. So it is not over 2TB. If I click "Yes" the email is saved to the "Inbox" of the shared mailbox. Why can't I save it to the "Drafts" folder? I can open the "Drafts" folder and retrieve older drafts, so that has not been moved or deleted. What to do?