venta

Some (probably most) servers do, but there are no guarantees. There cannot be any guarantees since it is conceivable that there is some kind of circular routing happening - if I send a mail with a return address pointing to a mailbox which is full, to a non-existant mailbox, the mailserver will attempt a delivery passing it to destination MX for routing onward, the destination MX doesn't have access to the final destination user set so accepts the mail to pass on to the destination mailbox. When, say, the destination machine dials in the MX attempts delivery to discover the mail won't be accepted by the destination host. At this point it should send a non-delivery notification to the return address. The non-delivery notification will fail, since the return address mailbox is full, what happens? The mail has to be discarded silently (it *may* get routed to an admin mailbox somewhere where human intervention *may* solve one or other of the problems, but it *may not*). The internet relies on these kinds of protocols not clogging up the whole network, there have been a few incidents where such routing cirlces caused significant problem to parts of the network.