Home Forums Networks & Cross-Platform Networking Apple Mail not displaying properly

Recently I did conducted one of my rare drive defragmentations. I used TechTool Pro. Since then, Apple Mail on my iMac displays images as a blank space with a small blue question mark in the centre.

I've looked around and the most common response seems to be ensuring that "Display remote images in HTML messages" is ticked under mail preferences. It is. Also, I conclude that the issue is with my Mail application, not a remote site, as the images display correctly on my iPad.

Cocktail has a function under System>Databases called "Rebuild Mail's Envelope Index". Does this sound like a reasonable next step, or is there something else to do first?

Are there any cautions to be noted with rebuilding the index?