We have a client that uses Outlook 2007 for their mail client, in house Exchange 2010 server, and has a mailbox size limit on the server of 10gb. They use one mail account on 3 machines. Periodically in the middle of an email, they attach a file and get the message that mailbox has exceeded size limit. The mailbox size is only about 3.4gb. (Verified in Outlook as well as Exchange) They then have to copy the body content of the new email to the clipboard, restart Outlook, paste the body, insert the attachment and then send the email without the error.
I have seen the post https://support.microsoft.com/kb/832925?wa=wsignin1.0
and it talks about setting the registry keys for file sizes, but the regkey HKEY_CURRENT_USER\Software
Microsoft stops at Microsoft. There is no Office key.
If the issue were too large of a OST file, then why does it allow them to close Outlook, re-open and then send without issue, not to mention happening on 3 computers?