My previous job was at a company that used Exchange for email and Notes for lots of web and some client apps. I noticed this occurred when HTML formatted emails were sent that contained long text strings, but never did the diagnostics that you have done. The solution as you stated is to add a line break regularly. Doesn't make any difference to the display of the HTML (as long as you don't break within tags).
My previous job was at a company that used Exchange for email and Notes for lots of web and some client apps. I noticed this occurred when HTML formatted emails were sent that contained long text strings, but never did the diagnostics that you have done. The solution as you stated is to add a line break regularly. Doesn't make any difference to the display of the HTML (as long as you don't break within tags).