My observation is some email that originate from phones using the apple logo appear to use apples very special text and it dies not wrap. Everything else in a HTML mail should wrap at the windows boundary, unless the HTML is embedded in a table and it has it's width set far to wide. I see this usually with corporate stuff and others that use MS office to build signatures that are really malformed. But you can not tell an MS office user their choice of HTML editor is rubbish they paid for Microsoft because it is golden.

Thunderbird will by default wrap plain text as 80 columns just as the old IBM3270 terminal did, so perhaps try reading those emails as plain text instead of HTML.