Microsoft Exchange - Host Unreachable...Que significa?

Alguien me podria explicar que significa, que en los correos que estan detenidos salga HOST UNREACHABLE-
Gracias...

Significa que no encuentra la máquina a la que tiene que acceder, o está mal escrita o no existe

esto significa que en la salida de tu puerto de un routeador no puedes transmitir datos, ocea que si por ejemplo estas utilizando la salida de un serial en un router para transmitir datos a un host y te aparece: destination host unreachable significa que no as definido el rango de datos que quieres mandar.

Simplemente hay un router en el camino que no tiene ruta hacia el destino.

He aqui un abstract del libro TCP ilustrated capitulo 3 IP Protocol sub tema IP Routing:

IP routing performs the following actions:
1. Search the routing table for an entry that matches the complete destination IP address
(matching network ID and host ID). If found, send the packet to the indicated next-hop
router or to the directly connected interface (depending on the flags field). Point-to-point
links are found here, for example, since the other end of such a link is the other host's
file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/ip_inter.htm (6 of 19) [12/09/2001 14.46.37]
Chapter 3. IP: Internet Protocol
complete IP address.
2. Search the routing table for an entry that matches just the destination network ID. If found,
send the packet to the indicated next-hop router or to the directly connected interface
(depending on the flags field). All the hosts on the destination network can be handled with
this single routing table entry All the hosts on a local Ethernet, for example, are handled
with a routing table entry of this type.
This check for a network match must take into account a possible subnet mask, which we
describe in the next section.
3. Search the routing table for an entry labeled "default." If found, send the packet to the
indicated next-hop router.
If none of the steps works, the datagram is undeliverable. If the undeliverable datagram was
generated on this host, a "host unreachable" or "network unreachable" error is normally returned to
the application that generated the datagram.