User-agent line not appearing in email

[EE]

I set "mailnews.headers.showUserAgent" to true, but the user-agent line 
appears only in the headers for newsgroup messages, not for emails.  Why 
is that?  The "mailnews" setting prefix should apply to both, not just 
to news.  It is not an extension causing the problem, since it occurs in 
safe mode as well as normal mode.  Is there any way to get the sender's 
user-agent to appear in emails?
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