receive emails but not the attachements

[lajoiem]

We have Exchange 5.5 SP3.
When people from the outside of the company sends us emails with attachements, it may work just fine but often it may take up to a week for the mail to get throught and sometimes it doesn't get through at all. When this is the case, the sender receives the following response :
_________________________
This Message was undeliverable due to the following reason:

Your message was not delivered because the destination computer was
not reachable within the allowed queue period. The amount of time
a message is queued before it is returned depends on local configura-
tion parameters.

Most likely there is a network problem that prevented delivery, but
it is also possible that the computer is turned off, or does not
have a mail system running right now.

Your message was not delivered within 4 days.
Host famili-prix.com is not responding.
_______________________________

When the same user resends the email with no attachments at all, everything works fine and the email gets in on time.

Do anyone have an idea ???

Thanks in advance!