Q Mail is misbehaving. When I receive formatted messages, for instance in HTML, much of the content is missing. Also when I send a Word .doc file it is shown as being a Missing Plugin. What is going on and how can I fix it?
A These are two different issues, potentially linked through a common reliance on MIME typing of message content.
Missing items in received messages are commonly linked images and similar content. If the embedded link cannot be accessed, because your Mac is disconnected from the Internet, the link is broken, or Mail is blocking the connection, then the content will be missing.
Try viewing the message in source format, by saving it as ‘Raw Message Source’, then opening it using a text editor. If you then see MIME-encoded chunks, you can save those out and decode them using Stuffit Exapnder. Files attached to messages should be recognised using the MIME type dictionary within Mac OS X, and in the case of Word .doc files should read
MIME Type: (null), so that they are transferred as binary files.
Hover the pointer over the enclosure and a floating window should give details of the MIME type recognised. Sometimes this dictionary becomes corrupted, or individual entries may get out of kilter, and enclosures will then be attributed to erroneous types, as you have seen.
Default Apps (alias RCdefaultApp, free) is a System Preferences pane that gives you full access to the dictionary, and several other useful system-level databases. Its only danger is that you might fiddle with these dictionaries needlessly and rot up the Finder and various applications, so be careful how you use it. Sometimes issues such as these can be cleared by rebuilding mailboxes, or even after a simple restart.
Further details and suggestions are here.
Updated from the original, which was first published in MacUser volume 27 issue 3, 2011.