Returned email

I am getting the following error message when sending  some emails to multiple recipients :-


"Unable to deliver message to the following recipients, because the message
was forwarded more than the maximum allowed times. This could indicate a
mail loop."

My questions are
a) Where is the maximum allowed times set? is it an Exchange setting, Outlook setting or set by the ISP.
b) What is a mail loop, and how can it be detected and fixed?


Thanks in anticipation.


Alan