[Exim] header_sender issue

Hi folks,

I was having trouble with my Exim not letting me send mail from some
accounts (but not others, no idea why) and the reject log said "there
is no valid sender in any header line", so I commented out the
following line from exim.conf:

require verify = header_sender

Since I'm not sure exactly what this does, and it's probably
important, I'm trying to find out what it does... not having much
luck.

Am I committing the 8th deadly sin by turning it off? How can I leave
it on and still send mail from those troublesome accounts?

-Josh