CiviMail error due to a word in the mailing - why?
I recently had a similar problem (with Moodle, not CiviCRM, on a shared host) giving errors when a particular word was in the content that was being submitted by the user. It turned out to be triggering a mod_security rule, resulting in a 403 error. Whitelisting that rule resolved the problem.

Edit (by Laryn): Here's the mod_security rule (#300016, "Generic SQL injection protection") that was triggered in this case: