[exim] message_size_limit

I do not have this option set in my configure file,
and it was my understanding that unset meant unlimited.

However I just had a message "bounce" 552 message size exceeds maximum permitted
SIZE=52571183

yes, it was huge, but I don't have this set? The docs indicate default is zero
(unlimited) but the above suggests otherwise. Am I missing something?

Thanks,
Jim