Some desktop clients corrupt HTML coding on draft/publish

First, my (test) post as sent to Draft or Publish:

<p><span class=”dropthecap”>N</span>ow is the time for all good men to come to the aid of their country. </p>

Second, the same thing as it comes through using some, but not all, desktop or browser-based clients (slashes added because the foul-up didn’t display here as it does on my WP Write page):

&/l/t/;/p/&/g/t/;/&/l/t/;/span class=”dropthecap”&/g/t/;/N/&l/t/;//span/&/gt/;ow is the time for all good men to come to the aid of their country. &/l/t/;//p/&/g/t/;

I’m aware of a similar problem from a year or two ago, the fix for which was to get one’s ISP to update PHP to 5.2.9. cPanelX on my ISP’s server reports its using PHP 5.2.13. Nevertheless, I installed the LibXML2 plugin, activated it and ran a test. The problem remained.

This corruption problem does not rear its ugly head in w.Bloggar in Windows or in Lekhonee in Kubuntu Linux.

The problem does occur in Scribefire on either platform, and in Blogilo and another desktop client whose name I can’t recall on Linux (I tried several).

Apologies if I’m bringing this up in the wrong place. Given all the variables involved, and the fact some clients don’t have the problem, I’m not sure where to go for help.