Some incomming mails are Bounce

when i send a mail, hotmail to our mail server, sometimes mails are not transfered (mail bounce) ( our customers said)but sometime mail came very fast into the out mail server

(our internet line bandwidth is 32kbps)

what is the probles. Is it our server problem or network problem.

Subject: need more information

Are there any errors in the log? Are accepted and rejected messages from the same sender? We really need more information for troubleshooting mailrouting.