I was pulling my hair out on Wednesday trying to figure out why certain inline images in an email I was generating would not display in Outlook, whereas others would.

What’s more irritating is that all of the images worked in Outlook Web Access; only the desktop client of Outlook wouldn’t display some of the images.

The problem is that something like this is really hard to troubleshoot. After going down many different paths–such as assuming that I was attaching the image incorrectly, or perhaps that Outlook didn’t like spaces in its inline attachments, and so on–I finally found the solution.

Outlook was recently patched to flag image references that have a src attribute with a capitalized CID. Thus, changing < img src = "CID:myimage" > to < img src = "cid:myimage" > fixed the matter.

Oh man. That’s frustrating. So I’m blogging about it in hopes that I might save someone who googled the issue like I did from wasting a couple hours of time.