[exim] Is there and logical reason to reject mail from: <> ?

ok - I've set up sender callout verification and I have a friend who
can't send me email. His service provider rejects all messages "mail
from: <> with 550 bogus sender.

I tried to reason with him and got no where.

Is there any logical reason that someone would reject <> as the from
address for everyone? Or is this just nuts?