Postfix problems

[Mike]

when i send mail to freebsd.org mail lists using postfix ralayed through 
my isp, freebsd.org doesnt respond.My posts dont show up on the lists 
and account requests are not processed.

If I use mozilla mail through the same isp: smtp.comcast.net things work 
as they should.I would like to set up postfix so I can use it as my MTA.

I am not sure where the problem lies.Is it postfix or my isp?

Thanx,
        Mike