Since a busy site mailer will normally result in use of alternative MX records if they are there, it allows the relays to absorb short-lived peaks of traffic to a site. Furthermore, they introduce some problems of their own. These are not new, but may now be considered to have greater relative importance. This has become an issue with the increasing volume of traffic, and mail traffic continues to increase. Longer-term failure, or changes to naming or topology at a site, result in messages timing out after some administratively determined interval, possibly with warnings to the originator during that interval.
Users now expect greater immediacy and are probably better served by an immediate report of failure.
The inevitable failure is then reported as having occurred at the relay, and a message originator understandably finds this unhelpful. Anti-spam measures are made more complicated. In particular, there are almost certain to be loopholes wherever a site's own defences differ in detail from those implemented at the relaying system. Use of relays for fall-back consumes resources, requiring the provision of hardware, operations and support. The two relay tasks which will persist are for X.400 interworking.