no such node
unrecognized node
node is unknown
name unknown
unknown addressee
no such user
invalid user
not a registered receiver
cannot receive
can not receive
send to
send mail to
has left
has transferred
on temporary leave
on leave
exceeds gateway
not in ems
error parsing
record too large
syntax error
invalid item
cannot accept
unbalanced
host unknown
user unknown
> unknown at <