The reason is that people don't like having their e-mails taken as evidence and used to build a case against them. The TIFFs are probably the result of a bulk scanning of lots of paper that was seized.

You could use a search engine for this task. It may even be worth getting your law firm to buy one if this is going to be a regular task. visit http://www.searchtools.com/tools/tools-perl.html for a list of Perl related search tools and other search related resources.

I use the Verity K2 search engine. It is big and very expensive but it works well.