Microsoft Outlook has a reasonably common, yet very generic error:
There are a bunch of reasons that can cause this error. Often, the ‘solution’ is to change the email from HTML to Rich Text or Plain Text, and move on. Or, copy/paste the entire email into a new email and move on. Neither of these are workarounds of course.
Dig a bit deeper on the web, and you’ll find some wackier reasons – the size of the logo in your signature for example. There was also a version of Exchange 2010 that caused the issue, but that was resolved.
I was running into this problem on a regular basis, and spent many hours trying to come up with a reason for the issue, and why only certain users had the issue, and on certain emails. I couldn’t reproduce on my own PC with the exact same emails, yet it was 100% reproducible on theirs. Also, if I removed the image from the user’s signature it fixed it – and it didn’t matter what image I put back in, it was always broken.
It wasn’t until I cried out for help on Twitter, and Christopher Kusek came to the rescue. After looking at a few other ideas, he pointed me towards hotfix KB3042197 with the lengthy title “ Graphics file attachment grows larger in the recipient’s email message after you change to a high DPI setting”
After reading this, I did some testing. Amazingly, the problem only occurred when the PC’s DPI setting was above 100%. Amazingly, when DPI is 125%, 150%, 200% etc, Word from inside Outlook resizes images in the HTML code based on your DPI setting. Some combination of HTML emails back and forth, and this resizing would cause the ‘Cannot send this item’ message.
Setting the DPI to 100% wasn’t enough either, as it would make the contents of the screen too small for some users. The registry setting mentioned on the KB article ‘DontUseScreenDpiOnOpen’ and setting the value to 1 (true) actually fixed it!
After some more testing, this single registry setting which you’d think would be on by default was rolled out, and the ‘Cannot send this item’ messages were no longer appearing. At least not for this particular problem!
TLDR: If Windows is set to > 100% DPI, the MS Word editor inside Outlook 2010 and Outlook 2013 will resize images by default and this can cause weird stuff to happen.