I just finished one of those late-night phone calls, one that also happened to interrupt my sleep. The culprit? Microsoft Outlook was blocking access to a file attachment that a user needed access to.
For the reasoning behind this functionality in Outlook, William Kennedy, the General Manager of Office Communication Services at Microsoft, has an explanation in "Blocked attachments: The Outlook feature you love to hate".
The kicker here is that the blocked file was a ".mat" file, which Outlook blocked on the assumption that it was an "Access Table Shortcut" file instead of the MathWorks MATLAB file that it really was.
William Kennedy's article describes a few alternatives for sending such files so that they do not get blocked, but these are of little use once the file or files are already sent and need to be accessed on the receiving end.
The quickest and easiest way to fix this is to download and install Slovak Technical Services' Attachment Options plugin for Outlook. This free adds an "Attachment Security & Options" tab under Outlook's Tools/Options, where chosen file extensions can be essentially "unblocked" by moving them from "Level 1" to "Level 2".
The alternative is to edit the Registry directly. See "Opening Attachments Blocked by the Microsoft Outlook E-Mail Security Update" at Slipstick Systems for the details. (All the usual warnings about editing the registry apply.)
Here are some other pages at Microsoft that may be helpful: