hxxp://imgur.com/JwtuB9Y
See the attached image. This is a very recent issue, but I had 6 users across 6 different locations all randomly have no access to send/receive email. I could not download their mailboxes, folders would not synchronize. The problem was ESET.
Key words for Google to find this post in hopes it helps someone else:
OST PST download fail cached exchange 2010 outlook 2010 disconnected from server unable to send email