dear all
i have a project on a soho company with a frame relay connection to internet (accross local isp).
the link to isp used 64k CIR with cisco 3640 as router.
the link was unstable sometime it can't ping to the isp router or even it could the time was to long (4-8 seconds).
and on another time (on the same our but on another day) it takes a short time.
the major problem with this condition is a duplicate mail on branch that receive mail from internet, they could take 70 duplicate mail from a single mail that send to them.
i thought this could be happen because of the tcp proccess on smtp that couldnt finished because of the bad connection to our isp, so the mail server retrying to send the mail until the process finished
all i want to know is how it could be happen (the bad connection) is it because the bad link from our frame relay provider or because the excessive trafic on the link or because another reason ?
and the second question is how can i check the main problem and to fix it  ?

thanks