The dreaded Sender: line

I need someone more familiar with RT to check me on something.

Mail::Internet appears to add a Sender: line to all mail it sends. When RT
calls $MIMEObj->send(), Mail::Internet’s sub _prephdr gets called, which
includes the following:

foreach $tag (qw(From Sender)) {  # Sender is deprecated

As far as I can tell, the only way to get mail out of RT without a Sender:
line is to use sendmailpipe, which uses $MIMEObj->as_string, which avoids
the _prephdr call.

Can anyone either verify this, or preferably demonstrate a flaw in my