Using classic Outlook for Windows in business environments
This is an incredibly disingenuous response - and has been WRONG for the past 20 years in all versions of Outlook since at least 2001.
The simple fact is that Outlook is incapable of trimming its headers properly, with the result that the length of the lines in the headers (which do not appear to be trimmed at all) overflow. This will happen in any long thread of replies.
Microsoft has been repeatedly told of this bug - and has repeatedly ignored it.