Problem recently started occurring when an Outlook 2011 client attempts to send an attachment over 1MB connected to Exchange 2007, an error is returned, the email is not sent and is saved into the Drafts folder. Error code 18596 "HTTP error. The resource is not found."
In Outlook > Tools > Account > Advanced > Server tab > Microsoft Exchange Server is changed to its internal hostname and SSL is unchecked and saved, the problem is temporarily resolved. Once Outlook is restarted, the Server field is repopulated with its Autodiscovery address "https://server.domain.com/EWS/Exchange.asmx", SSL is re-enabled and the issue returns.
I've Googled this and the top 5 results point to posts in this forum, however, EVERY link 404s. Searching this forum for "18596" returns no results. What's up with that?