There are no such things as an HTML Email compatible with *ALL* mail clients. You need to list them. *ALL* mail clients equals thousands of different Web and Desktop applications. Multiply that by the number of different OS and different Browser Versions and you get an astronomical number that nobody on earth can test.
The only way to guarantee that is to use plain text. HTML is not part of the email spec, so not all email clients will handle it. Some will ignore it, some will display it as plain text (including all the tags), some will work more or less well in rendering it. The only thing specified in the email RFC (rfc2822) is text.
Well, if he's able to type text here, he's able to type it in his newsletter sending tool... So there must be some HTML layout involved. no?