>Why does their e-mail shut down?
 
Probably because of hardware, software, network, or human
errors. ;-)
 
Why does anything fail?  You must do both a component
and system analysis, along with whatever service agreement
might have been made (if any).
 
As far as I know, no one ever made a 100% guarantee that
e-mail was reliable, available, or confidential for that
matter, or the networks and domain name system thereof,
including, but not limited to, acts of war or god.
 
(Yup, I know that ARPAnet was designed to survive in
some manner of war-time.)
 
/Pete Weiss at Penn State