Thanks for that, but no; I don't think mine's related to that one. I'm not running any sort of request filtering, and I can receive emails from all sources I've tested (Outlook from two locations at work, GMail, another private email server). I can receive emails with attachments without issues, and every client can send emails to any recipient, unless it has an attachment.
I though something might be wrong with the temp folder, but it doesn't appear to be.