Microsoft says that Outlook freezes “by design” and that they are not going to be working on this issue. Below is their explaination and proposed solutions:

Issue Statement:

Outlook contacting server for information when replying/forwarding and printing emails with HTML contents and images.
The behavior that we are experiencing is expected when we have Outlook 2007 and above which does not cache message references (hyperlinked images in our case). Reason behind this is an expected behavior is Outlook 2007 and above versions do not keep the reference data of message/email body. The time it points to references data from specific location which is mentioned in hyperlink, which is nothing but advanced HTML content which renders based hyperlink. Unless the entire content is downloaded Outlook.exe will wait, once entire content is fetched in email Outlook.exe process will respond back. So the hang that is experienced is moreover an expected behavior when we have such advanced HTML content. This behavior with advanced HTML content is known to Microsoft since product is designed to wait until the content is loaded 100%.

Symptom:

Outlook contacting server for information when replying/forwarding/printing emails with HTML content.

Cause:

This is a known issue when you are dealing with HTML emails which have the images on a different server and are not embedded images.

When you open HTML emails which have images that are stored on another server Outlook will contact the server to download the images every time you open the Email. It won’t save it locally as the image is not a content of the Email. The emails only have links of the images which is actually on another server.

So when you forward or print Outlook will contact the server once again which ultimately will take some time to download the images.

In Outlook 2010, default and the only rendering engine is Word, and since Word is not a browser each image is considered separate from any other (a unique link). This is unlike a web browser which treats images like a pool of resources and will reuse the same one over and over if two tags share the same link path. So whereas a browser only needs to pull the image once, word will pull it for every image. In addition, word don’t guarantee rendering of content designed for web pages and not email.

Reading the email is OK because we can download images in the background (it is a synch). But to forward or print, we have to pull the images synchronously because we need the download to complete before we can proceed to the next step. So the reason it only occurs on those steps is by design.

Resolution:

This is a by design issue.
We have listed a few workarounds below:-

Options:

Uncheck the option in Trust Centre “Don’t download pictures automatically in HTML e-mail messages or RSS items.” which you have already tried and it is working for some emails but not all.
You may create a Registry Key as mentioned below which gives you the same behavior as the first step:

HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Common

Dword: BlockHTTPImages

Value: 1

You can open the Email in Web Browser by opening the Email and going to Actions -> View in Browser.
In Outlook 2010, under File tab, Options, locate: Replies and forwards: there is the option of four ways of forwarding email:

Fix:

File -> Options -> Trust Center -> Trust Center Settings -> un-tick the ” Don’t download pictures automatically in HTML e-mail messages or RSS items.